Joining Plywood

[Mark W Pugh]

So, I have joined plywood for doors in the shop. They seem to come out great, and the primer/paint covers the seam well. We, sorta. After a short time, the seam begins to show. Not a big deal, but I need a technique on how to prevent this.

I though of chamfering, ever so slightly, the joining edges of the viewable piece. Then, fill it in with.......? I thought of drywall compound, which may be too wet. How would you guys do this?

Thanks

ps this will be painted.

[Victor Robinson]

What kind of joint is it and how big is the seam? A photo would help a lot.

[Steve Jenkins]

I have edge glued a lot of veneered panels to make them bigger or for a match and always use a spline rather than biscuits as it gives a continuos reinforcement to the joint. However for paint grade I would suggest plain mdf rather than veneer core ply. There is no grain to telegraph and it's cheap enough that you shouldn't have to go to the trouble of joining pieces unless you need wider than 49".

[Martin Wasner]

If I've got a wood top that I can't get out of a single piece because of length I'll do the splice with a piece of solid in between the two sheets. I'll pocket screw from the sheet stock into the solid on both halves. I've had good results with that method, but I don't do it where the pocket screws aren't hidden.

[ian maybury]

Hi Mark. Having done a fair amount of car body repair in my youth i tend to like bondo type polyester base fillers - and have used the Unibond wood filler well down this page http://www.handytools.co.uk/acatalog/Handytools_Catalogue_Repair_for_Good___Metal__Ston e___Plastic_1741.html (a UK brand - don't know about the US but there's likely equivalents - think the product was branded 'Plastic Padding' until pretty recently) on hardwood and ply at different stages. Much as you describe - chamfer the face corner slightly to provide a shallow but very wide angle V (maybe 1/32in deep x 1/2in wide each side) for the filler to sit into - this is the key since the thin feather can move with the wood if required. It'll add a tiny bit of strength to the joint, but realistically the panels need to be well joined independently of the filler using a spline or similar. Rough up any shiny glue surfaces too to assist adhesion, best not to use epoxy as polyester doesn't bond well to it.

The Unibond product is sold as a 'wood' filler, but realistically i'd be surprised if it was any different from standard car body fillers other than the light colour. The trick is not to over apply it, and definitely not to create humps that will need a lot of sanding to remove. Wipe it on flush with the surface with a plastic spreader, then cut back the resulting very minor high spots carefully with an 80 grit block. (or you may cut through the first veneer) Wipe on another skim if needed, and repeat. Let it harden well (it likes plenty of warmth) before sanding as it can otherwise be inclined to clog paper.

It's obviously not a runner under a clear finish (even if the colour matches it looks a bit odd), but i have joints in white painted hardwood in the house done 20 years ago which to this day are invisible. No settling or sagging.

I'd imagine there could be a little risk that a cheap ply might move a hair if the joint is left too narrow/abrupt/without the V and feathered edge (as in be visible in light across a gloss surface over time), but a good wide chamfer as above so that it feathers out to nothing over a decent distance each side of the joint should pretty much eliminate that. Ply with a hardwood top veneer might (or might not) be a shade better if you want to be really picky....

[Jim Dwight]

I've used car type bondo for both inside and outside use and have had good results. It takes paint well and is easy to use. I wouldn't mess with the stuff sold for wood, I think it is more expensive and not functionally different from the kind sold for cars.

[Allan Speers]

Like Jim, I typically use bondo. I prefer it to the various wood epoxies I've tried, because it's easier to sand after curing. It's also a LOT less expensive.

It's no good for real wood, as it will crack over time, (it's not flexible) but since ply & MDF don't move, it's ideal for these. It even takes dye and stain somewhat, when required, or you can tint it with dry pigment.

[Mark W Pugh]

Thanks everyone. I guess I need to learn how to use bondo. Any good sites you recommend? Yea, I know, just google it. However, that give you all the bad stuff too.

[Martin Wasner]

Thanks everyone. I guess I need to learn how to use bondo. Any good sites you recommend? Yea, I know, just google it. However, that give you all the bad stuff too.

Just follow the instructions on the can. It's not rocket surgery