mdf exterior questions

[Jeff]

Hello,

It was recommended I ask about this here.

Project is a solar collector. Frame to be made out out of roughly

3/4" MDF 4" deep. Roughly 2' * 10'. The front is SunTuf glazing and the back is non structural. There will be a few cross braces. Collector will be roof mounted.

Questions:

1) Best sealer for this. Thinking Zinser shellac (bullseye), several coats. Also considered but not yet found are the two part marine sealers. Will the edges need extra treatment or will 2 or 3 coats be enough?

2) How to make corner joints. Thinking just butt joints held with something like titebound, also heard of using gorilla glue. Do I need to dowell this? Whats the difference between titebound and titebound II?

3) Sides need to be 10' but material is 8'. I'll need to joint two pieces. Thinking either a lap joint two rabbits together or just butting this up and screwing a mending strip to hold it together. Perhaps the Gorilla glue would be better here with it's expanding/filling tendancies.

4) Exterior Finish. I have access to free Sherwin Williams house paint (mistints). Good enough or should I look at an epoxy, and what kind/brand. This is going on the roof so it will take a thermal beating.

Hope this isn't too far out of the range of finer woodworking projects/techniques that I see posted here!

Jeff

[Doug Miller]

Unless you live in a place where it never rains, you should reconsider your choice of MDF. Marine plywood would be a much better idea.

Paint. Over marine plywood.

Think again. No matter how many coats of shellac you put on it, it's not going to stand up to rain.

Paint. Over marine plywood.

Screws and corner blocks would work a lot better -- but you still need to be using plywood, not MDF.

First, it's Titebond, not Titebound.

Second... Titebond is not water-resistant, and is totally unsuitable for outdoor work. Titebond II is water-resistant, but not water-proof. Not that this matters, since MDF isn't waterproof or even water-resistant either.

Screw a mending strip on there, or nail a 1x4 across the joint. Caulk it. Forget the glue.

You shouldn't worry about the thermal damage nearly as much as the water damage. Use a good grade of exterior or marine plywood, and don't worry about what kind of paint you use.

[C&S]

Ditto that. Water+MDF = oatmeal

[gw]

I hope you mean MDO - Medium Density Overlay. MDF will not work - I don't care what you seal it with. MDO is used in the sign industry and is intended for outdoor use. MDF will not tolerate *any* moisture.

See above. Epoxy would be a good bet on edges and joints.

If not structural, butt joints should work. I would add stainless screws and use waterproof glue. Titebond I or II are probably not suitable. See epoxy above.

Either would work with proper glue.

I would stick with epoxy or marine finishes unless you want to get up on the roof and paint it every year.

[RayV]

Go here:

if Morris doesn't catch this post ping Morris Dovey

[Clint]

Others have already commented on using MDF outdoors, so I'll leave that alone. But I was going to comment on 3). While Gorilla Glue may expand to fill a poorly done joint, the expanded glue is NOT structural. So don't count on it to hold just because it filled the gaps.

Clint

[Pat Barber]

There are several exterior grade MDF products:

MDO plywood might be more available in your area.

Check with sign makers and painters.

Jeff wrote:

[Teamcasa]

"Jeff"

I'll chime in and say forget the MDF and get MDO or exterior grade or marine plywood. Epoxy - yes, Titebond - no. Shellac - no, paint - yes, Butt joints - no, pocket screws, dowels, tongue and grove yes.

Dave

[Morris Dovey]

|

link

| and if Morris doesn't catch this post ping Morris Dovey

Jeff came here from alt.solar.thermal at my urging. As a panel producer, I didn't want to be the one to butt into an ongoing thread to rain on anyone's parade (er... MDF) - and I knew he'd get good advice here. Thanks to all for helping!

-- Morris Dovey DeSoto Solar DeSoto, Iowa USA

[Lew Hodgett]

Teamcasa wrote: > "Jeff" >> Project is a solar collector. Frame to be made out out of roughly 3/4" >> MDF 4" deep. Roughly 2' * 10'. > I'll chime in and say forget the MDF and get MDO or exterior grade or marine > plywood. Epoxy - yes, Titebond - no. Shellac - no, paint - yes, Butt > joints - no, pocket screws, dowels, tongue and grove yes.

MDO and/or exterior ply - yes.

Marine ply - no, a waste of marine ply.

Epoxy - absolutely.

Butt joints - no way. If necessary to achieve 10 ft length, sister two (2) layers of ply with staggered overlapping butt joints using epoxy.

Box joints with dowels for corners approaches bullet proof.

Couple of coats of a good paint.

Lew

[the_tool_man]

Hi Jeff:

As others have said, don't even consider MDF for an exterior application. I used to recommend MDO for this sort of thing. However, as one who has used MDO for several outdoor projects, I'd recommend against that, too. MDO is made by facing plywood with waterproof paper on one or both sides. It is great for signs that are painted. However, if anything gets screwed into it, water can find its way into the plywood. The waterproof paper then traps the water inside where rot quickly develops. Before long, you have two layers of paper with nothing between. If you are very careful about protecting the edges, and can avoid using any fasteners that penetrate the paper layers, and are careful to paint it and keep it maintained, then MDO may work fine.

I would use a quality exterior plywood (not marine - overkill), and paint it. You might also consider pressure-treated plywood, which should hold up better if you can find some flat enough. If you use PT, make sure it's dry before painting.

Use either epoxy or polyurethane (Gorilla) glue.

Regards, John.

Jeff wrote:

[Jeff]

I've got the MDF cut so at least some boxes are going to be made of it.

From alt.solar:

Well, it's all a bit late now. I switched off plywood just before I bought the materials, so 6 boxes will be made of this. It's really the copper and the glazing that cost money and they can be reused when the boxes fail.

Current plan is to build the boxes with MDF, prime them with Zinsser BIN and then Sherwin Williams house paint.

The boxes will be off the roof an inch and the Suntuf will lap over each side and the bottom so drips won't run into the seams. The top edge will have a bit of plastic bent over the width of the box and attached to the SunTuf. That leaves the holes/ slots in the box side where the header pipes go through. Perhaps a slit funnel... and some silicone...

My understanding is that marine has no voids.

Highland Hardware suggested Titebond III for the glue. Couldn't find epoxy finishes there, they have a lot of stuff, so I may have missed it. But I asked.

I'm leaning toward a rabitted joint with a mending plate inside. Or perhaps a miter joint =/=

I think that's overkill for me.

Jeff