Pinewood Derby 101 - painting a car and getting a smooth finish - need help

usenetdg wrote...some questions about finishing the car.

First off, I've never seen a really nice looking brush-on paint job on one of these cars. My son brushed on his first one, and was very disappointed with it, so I cleaned off his paint with lacquer thinner (well-ventilated area with a fan, and use gloves!) and taught him how to spray it. At that age, his little hands couldn't hold down a spray can button very well, but this was easily remedied with one of those cheap aftermarket gun-like squeeze handles.

Sand the bare wood to 180 or 220 grit. Use a light touch and a lot of back-and-forth strokes with the grain. Start with 60 or 80 grit, and don't skip any grits. Shouldn't take more than two or three minutes per grit at the most, and probably less. When you're finished, clean off the dust with a rag at least; a tack cloth or vacuum is better.

Run a fairly stiff wire through each of the axle slots. It needs to stay in the slots, so you can lift and turn the car over by the wires, but it shouldn't be so fat as to "waller-out" the slots and make the axles loose. You want to be able to suspend the car on the wires between a couple of bricks or something, spray the bottom, flip it, and spray the top.

You will need a couple cans of paint: whatever color he wants, the primer, and optionally a can of clear. Avoid multi-color schemes, pin- striping, and other complexities for his first year. Decals after, though, are great!

I like Krylon, but we used Rustoleum products with equal success. It's a good idea to stick with a single brand, though, as mixing brands can sometimes cause crackling, especially with the clear coat.

Spray a very light coat -- a dusting really -- of the (wood) primer. You should still be able to see the wood grain through the coat. Wait five or ten minutes and spray another coat just like the first one. Another wait, another coat, and the wood grain should be completely hidden. Let this coat dry for about 30 minutes, and then lightly sand it with 320 or 400 grit. Don't go through the primer to the wood, just sand the primer. If you do sand too deeply, shoot another coat and sand again. You want a smooth finish of primer.

Dust it off, and shoot the first color coat. Resist the temptation to shoot a heavy coat, trying to go for the glossy look. Again, you should be able to see 50 to 20% of the primer through the first coat of color. Again 5-10 minutes wait, and shoot coat number two. This coat should just about completely color the car; not much if any primer still showing. Too light is better than too heavy. If much primer is still showing after coat two, wait your 5-10 and do another.

By the way, other than after the final primer coat and now, the waits between coats should be only five to twenty minutes max, just until the paint is dry to the touch. If you wait too long, overnight for example, you increase the risk of crackling the paint.

For a really smooth look, let it dry 30 minutes to an hour this time, and then sand very lightly using 600 grit wet-or-dry paper with just a drop or two of water to keep it wet. Wipe off the slurry with a slightly damp rag. An old T-shirt is perfect.

The condition of the surface at this point should be baby smooth, with maybe a few very small spots of primer showing. Shoot the final color coat. This coat should be *just heavy enough* to wet (not soak) the entire surface. Be sure to do the bottom first and then the top.

After the final color coat, your son will probably be pleased as punch with his finish. If so, I'd say you're done. If he really wants to go for the extra-fancy-super-high-gloss look, he can add a coat or two of clear. However, this introduces the greatest risk of crackling, especially if any of the previous coats have been too heavy, or if the clear is applied too heavy. The clear should be applied just like the final coat of color.

Good luck! These are some good memories.

Jim

Finally, a question I can speak with authority about. You certainly aren't the first dad to "get into" building a pinecar. The parents always build the cars for the younger boys, and some boys never get to build a car because Dad can't let go. My son and I built 4 cars including one that earned best looking AND fastest out of 84 entries, and another that was fastest out of 30 entries so far. Our district race is next week and we'll see how it stacks up there. He built this last car himself, but I couldn't resist coaching him every step of the way. Now that the credentials and bragging are out of the way, your questions:

1. Dried paint is very light; don't sweat it. Go to Walmart and buy a bag of split shot fishing weights. Take the weights and your cordless drill with you to the race. They'll have a scale set up where you can adjust the weight of the car until you get it right. If the car's too heavy, drill holes in the bottom until it's right. If it's not 5.0, drill holes and force the split shot into them until it's over 5.0, then back down.

1. You didn't hurt anything. You'll just seal it with paint.

2. Get some finer sandpaper. About 320 grit or 400. The first 3 coats or so, sand until the bare wood begins to peek through. Don't worry about how the car looks for the first several coats. What you're doing is filling in the coarse grain of the pine with paint and sanding down the ridges. But your'll never get the finish you're looking for with a brush-on paint. It's not too late to switch to Krylon spray primer, then Krylon gloss enamel for the final coat. The primer builds up well to create a smooth surface. The enamel dries to a real nice finish. They both dry really fast, so you can sand and paint 2 or more coats per day, one before work, and one at night. Keep priming and sanding until you can't see the ridges of the grain anymore. Then put about 3 or 4 light coats of the final color on without sanding between. These can go on as little as an hour apart.

1. The spray-on clear coat adds a nice shine, but it doesn't make it look like it's dipped in plastic. Put it on over the decals to hold the edges down.

2. You didn't ask for this one, but I'll pass it along anyway. Get some jeweler's rouge and a felt wheel for your Dremel Tool. (You do have a Dremel Tool, don't you? If not, this is the perfect excuse to get one.) Chuck the axles up in your drill and let your boy turn it slowly while you polish the axle with the dremel. Makes mirror finish on the axle that's really fast. If you remove so much metal with sanding & polishing that the axle diameter is a little smaller where the wheel runs, that's even better.

Good Luck DonkeyHody

I know you asked for finishing advice, but I can't resist! (I had quite a few winners in my day :oP)

Most rules allow graphite lubricant (usually used for locks, etc) on the axles - use it!

I always made the car a bit under weight, then brought along some plumber's putty to get it RIGHT up to weight by the judge's scale.

Happy racing!

My boys have walked away with a number if high place finishes as well as awards for best design or best paint job. Here's what we do:

1. Drill a hole in the back of the car running the length of the car (but not all the way to the front). The actual length depends on how much material you plan to remove from the car. If you are going to remove a lot then two holes might be appropriate. The drill bit must be slightly larger than the cylindrical weights that you can purchase at hobby shops or the Scout shop.

1. Cut new axle slots so that they are parallel to each other and hopefully perpendicular to the car. An alternative to this is to just drill a new hole in the side of the "top" of the car (the bottom becomes the top) for an axle. Then drill out a hold where the pointy end of the nail will be. That way the judges can see that you used the nail and not a solid axle.

2. Cut the car to shape, then put a drywall screw in the bottom of the car. This is used to hold on to the car while painting. It also give you something to suspend the car from while it is drying.

1. Sand up to 220 grit.

2. Spray with primer using light coats.

1. Sand with 300 grit and repeat steps 5 & 6 until you have a smooth, paintable surface. We usually put on 2 to 3 coats of primer before we have an acceptable surface.

2. Clean the surface.

1. Paint. (We buy our final finish paint from the hobby shop)

2. Assemble wheels/axles and lube. If you get some graphite on your shiny new finish it can be taken off with a small amount of car wax.

1. Install weight as necessary. You can plug the opening for the weight in the back but we usually leave it open. I squirt a bit of caulk into the hole before the weight is installed to keep the weight from shifting.

Be sure you read the rules that your Pack uses. I broke our car while trying to cut the car. So I replaced it with a block of wood from a scrap piece of 2x4. That resulted in two problems. 1) The 2x4 was much more dense than the wood that came in the box so I ended up with a car that was almost hollow underneath. 2) we were disqualified because the rules vaguely stated that we had to use what came in the box.

Sanding sealer and lots of it untill the wood is flat and smooth. Then one extra coat of sealer for good measure. A slick finish is the result of work done before the final color coat goes on.

I have found a very good clear water based sanding sealer that dries fast. It is "Endro" made by these people:

William Lee

It is not his project, it is not your project, it is a "Father-Son project. work with him, let him work with you. The good results are in the time spent, not speed of car.

"Old Cubmaster"

Talk to Silvan about aerodynamic shapes for Pinewood Derby cars. ;^)

Barry

...

You have a couple of replies that mention the nails and nail slots in the block. Be sure that you know the rules of your pack and district race (if your district runs a race) regarding these modifications. Our district race is coming up soon and any car with re-drilled axle slots will not be allowed to win. (I will never prevent a scout from at least running his car down the track, since the parent is almost always the reason the car is ruled ineligible.)

Ken

Right on - however, it can be a good learning opportunity where the cub learns from Mom/Dad and then applies the lessons himself. At the same time, provide guidance and expert opinion is not the same as doing the work for the scout.

We are in the suburbs of Motown and many of our parents work at the car companies, including a few who work in the design staff or model making groups. Some of our Pinewood Derby cars would look fine on the floor of a car show and I am sure that some of them have been in the wind tunnel. What we did to deal with this is to have a parents "no holds barred" competition. Those parents who just had to show off could compete among themselves at the same time as having an enjoyable learning experience with their sons. BTW - none of the "show cars" won the actual races.

A definite.

If you have a lot of grainy patches and tool marks that are too difficult to sand out, one handy dandy trick that worked out perfectly was Bondo glazing putty. The smoother the underlying surface, the smoother the paint.

Good luck from me as well, and a final piece of advice: buy fresh new paint, and make sure the paint, the car and the air are all well into the acceptable temperature range before spraying it.

I still don't know exactly what caused our finishing fiasco, but it was ugly. The paint never cured, and it wiped off with little effort. We ended up brushing it after all.

It took second in the Pack in speed though. Not too shabby. :)

Some other tips:

If you have a drill press, use it to drill out the axle grooves before you do anything to the block of wood, while it's still square. Boring out these holes perfectly perpendicular with a machine saves cracked bodies and crooked axles later.

The car that wins is almost always one that weighs 5.0 ounces or just weesny bit more. Due to the nature of digital scales, you can produce a car that weighs 5.004 or some such and still have it come in at 5.0 on the scale. Toward that end, we like to make the car too heavy on purpose, then drill out some weight a bit at a time on race day until it comes in legal. (Then after that, DON'T SO MUCH AS BREATHE ON IT.)

I'm not talking about re-drilling the slot, just using a drill bit to pave the way through for the nail, removing a tiny bit of material from inside the slot itself. Nothing sucks worse than spending umpty hours on a car, then having the fibers compress and split when you drive the nails in. It's also much harder to get them in straight when the car is full of curves than when it's a block of wood.

I don't think even our District people would complain about that practice, but I could be wrong.

For my son's car. We use about 3 coats of sanding sealer for a base. Then we brushes on a couple of coats of water based acrylic paint, followed by a gloss clear coat. Lightly sanding between the coats. I let my son do most of the work on his car.

For the Adult Class, I use about 5 coats of sanding sealer for a base. Then I apply 2 coats of spray enamel. I have posted some pics at alt.binaries.pictures.woodworking newsgroup.

Randy

Check with your particular Pack. Many will allow multiple weigh-ins.

Now you don't care how heavy your car is in an absolute sense. You only want it to be just legal according to the scale AT THE RACE.

If multiple weigh-ins are allowed, make the care heavy. Weigh it, step outside with a drill and remove a little, weigh it, remove a little more .... repeat until just barely legal (according to this particular scale).

I've seen Packs that use regular old postal scales (say a 5% measurement) and one Pack that usesd a lab balance (absolute measure to much better than a milligram).

By the way, paint may help a car look cool, but it generally doesn't make a car fast. Though I've sometimes wondered if putting graphite-embedded paint would help the around the axles. (Such paint is used for slides on farm gravity wagons.) Dang, did I wonder that out loud? Another "trade secret" bites the dust.

Good luck racing!

hex

-30-

I've done my share of drilling to get derby cars under the weight limit, but if possible, I recommend that you avoid this technique. It can damage both the finish and the wheel alignment.

Another possibility is to use small wood screws for the last bit of weight. Then remove one screw at a time until you're under the weight limit.

I'd like to thank the many who offered great suggestions in this thread, to which I'd originally posted a few weeks ago. The car we made wasn't great, but we learned a few "what not to do" things for next year.

The car is about to be weighed and impounded. Last night, I put the wheels on. I was really disappointed.

The big problem is that the car pulls sharply to the left. In fact, when pushed (i.e., not using gravity), it winds up going in a circle with a 10-12 foot radius!

I had put glue in the axle slots before putting the axles in, so I didn't want to mess things up worse by removing the axles. I tried pushing the front wheels a bit so as to bend the front axles to straighten the car out, and that may have helped a bit.

What is the best way to assure straight alignment next year? And any suggested alignment fixes for this year? I don't want to tear the axles out. Both front wheels have (guessing) 1/16 to 1/8 an inch of space between them and the body.

The other issue is wheel spin. I read somewhere that well-turned axles, properly polished and lubed with plenty of graphite/moly, should cause the wheels to spin for 20-30 seconds freely. I doubt ours spin for much more than 10 seconds. I have added PLENTY of graphite!

Our bottom line is we had fun working together on this (with me doing

Before you do anything else to next years block of pine, use a drill press to drill your axle holes in the block. If your rules require you to use the BSA slots, drill pilot holes in the slots to guide the axle as you insert it. After you're done, you can shape it any way you want, and your axle slots will be true to the (now nonexistant) sides of the block.

I'm not sure I have a good fix in mind for this year's axle problem. Maybe you could use a drill press with the bottom of the car clamped to a 90 degree fence of some sort. That might work if the bottom were still flat, but you'd be risking some pretty bad toe-in and toe-out if the sides of the car aren't square any more.

Good luck, Marcus

What is the purpose of the pinewood derby? Is it to see who's dad has the best tools? Are the kids allowed to do anything to the cars? :)

Ed snipped-for-privacy@snet.net