Polyurethane

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Working on a paint job on my Formica counter top (primer, granite spray paint, and polyurethane finish). This is the first time I've used Polyurethane and I'm getting different directions from different sources.
My question for anyone who would know:
Do I need to sand between coats of Poly?
I'm going for 4-6 coats, have the first coat on and dry, and am not sure if I should sand or not. The directions on the can say "recoat in 6 hours. If first coat is allowed to dry 48 hours, sanding is necessary between coats."
Does this mean if I don't wait 48 hours that I do not need to sand?
Thanks in advance for any help.
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On Wed, 10 Jan 2007 22:42:50 +0000, Hardcore Icon wrote:

Generally speaking do what the coating manufacturer says, not what some self-proclaimed expert says unless it's more conservative.
In the extant case, if you recoat in less than 48 hours you don't need to sand. I would anyway if I were going for a good finish--the smoother the surface the better the finish.

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Is there not the concern that the finish has to completely harden before sanding? I would think (w/ nothing to back it up) the longer the better. For the sake of the after-sand finish quantity/quality. Like multiply x 5 to be safe. Fives a good average number for things like that in general, any opinions? Quit asking questions, you're confusing us.
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I have read in here that (i)only sand is to worsen the finish, OR (ii)work in a polish regime,which may include some form of sanding to improve the finish. If you can recoat before the dry time w/o sanding, and you're not gonna super-finish, thats what I'd do.
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Remember that you may need to sand after the first coat if any grain is there to raise.
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yes, for a mechanical bond also after mfr rec'd time passed. My experience is if the wood is "rough" to begin with it may help between coats (specifically after spit coat), but that wood may not be worth improving. If its nice in the first place, sanding after the second coat and after gives little if any improvement for the amount of effort req'd.
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Polyurethane will yellow over time... I would consider polycrylic to cover your "granite spray paint" finish.
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On Wed, 10 Jan 2007 18:47:51 -0500, bent wrote:

If the finish has to completely harden before sanding then I'd start using a different product. The precatalyzed lacquer I've been using lately is ready for sanding in 40 minutes (per the manufacturer) but not completely cured for several days.
The main issue with sanding is whether it is hard enough to sand without prematurely clogging the sandpaper or otherwise misbehaving. I can't see where waiting a week between coats is going to gain anything.

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Thanks John!
Maybe I'll hit it just a little bit with some 220, it hasn't been 48 hours but if it will make a better finish then that's what I'll do. But the directions only said to sand if it's passed 48 hours so going by that I guess I don't *have* to sand.
Thanks again,
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Hardcore Icon wrote:
<snip> > The directions on the can say "recoat in 6 hours. > If > first coat is allowed to dry 48 hours, sanding is necessary between > coats." > > Does this mean if I don't wait 48 hours that I do not need to sand? <snip>
Yep.
After 48 hours, you no longer get a chemical bond so you have to sand to get a mechanical bond between coats.
Pretty much standard for LP.
Why not talk to the factory for latest update.
Lew
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Thanks for the info Lew!
Glad I found this NG.
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Try it by the manufacturer's recommendation, but don't be surprised if the second coat drags if you don't scruff between coats. Poly does not bite into the previous coat, it only lays on top, so the smoother the underlying surface, the better. I would not use 220 though. I'd just knock it down with steel wool or 3M pads. All you need to do is break the glaze for a little tooth.
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Exactly what I did Mike, and it worked great!
The 220 sandpaper didn't work that great but the steel wool did. Plus it made it easier to see where I've put the second coat. Helps to keep an even coverage.
It's looking great! A few more coats to go (takes a day to recoat, and being busy at work isn't helping).
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Watch out for steel wool filings and water-based finishes. It's a bitch getting rid of the rust spots.
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Steve wrote:
> Watch out for steel wool filings and water-based finishes. It's a bitch > getting rid of the rust spots.
SFWIW, that's why bronze wool is out there.
Standard boat finish equipment.
Lew
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On Sat, 13 Jan 2007 05:17:39 GMT, Lew Hodgett

As well as synthetic rubbing pads.
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you have to rough the surface up a bit. I like to put on a few coats, sand (well, steel wool) and then put on my final coat. The more often you sand, the better chance you have of getting a smooth final surface. But it is work and makes for a thinner coating.
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Great.
I've got all kinds of sandpaper and steel wool too just in case I need it, but going to try a couple coats without sanding and see where it leads me.
We'll see......
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Hardcore Icon wrote:

Duhhhh, do you suppose that's what it says?
Reason to sand: Air is not dust free; can see dust particles. Reason not to sand: In specs per the directions and surface looks perfectly fine, no dust, bubbles or other contminants visible.
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Gotcha!
Since this is not wood (it's a countertop that is painted to look like granite), whatever dust particles can't really be seen anyway. Good reason not to sand.
The main goal here is to get a real good thick protective coat. Just put the second coat on a few hours ago.
Looks good.
Thanks.
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