March 18, 2012

Spray painting over butt joins

Spraying over butt joins, especially with MDF, has always been a problem. The typical result is a seam or crack at the join which is quite unsightly. An internet search reveals some quite involved attempts to get around the problem. The most reliable solution seems to be to avoid them completely and veneer over the join, but it is unfortunate to go to the effort and expense of adding veneer only to paint it. Further, when one wants a roundover on the edges, veneer is not a real solution.

Understanding the problem

The seam is caused by the edge of MDF soaking up more than the surface, resulting in expanding after painting.

My first trial involved an MDF butt join where I wiped the surface of the join with polyurethane glue, then added two part bog filler over the top. I then applied another layer of each before brushing on undercoat then spraying with a can. When a seam came up I applied more layers or glue and bog, but it kept appearing. The seam was quite a bit more subtle than if no treatment were used, but still I'm aiming for a higher standard.

Trials in progress:

1. MDF with PVA glue - this is the base sample I'm doing so that I can compare the results step by step.

2. MDF with Polyurethane glue - does a stronger glue reduce the problem?
3. MDF with a groove cut with a router bit to hide the seam.

4. MDF with fibreglass reinforced filler over the seam, auto bog over.
5. MDF with a groove cut, then filled with fibreglass reinforced filler.

All of these were then repeated with ply. This example is with ply #5.



  1. You could always try the "tape and mud" technique that drywallers use. "Tape" with fiberglass tape, "mud" with fiberglass resin, feather and sand.

  2. Fibreglass tape! That's a good suggestion I didn't consider.

  3. I suppose it all comes down to the look you are trying to achieve, but a good rule of carpentry is to accentuate what you can't hide. A groove, such as the second picture, might just be the simplest and best solution in the end.

  4. I now have primed the samples, all 10 of them and at this point the ones with fibreglass reinforced filler are looking by far the best, and also the most work. With the grooves, that particular one is hard to get smooth. I may have been better with a shallow saw cut groove.

  5. I have never had issue with join lines showing through when using wood like marine plywood, l assume because its treated to stop moisture. Have you considered using waterproof sealer on wood?

  6. I'm about to run my own tests, hopefully very soon... before Tassie gets colder and more humid than it already is.

    Resin/wax impregrated HDF, and MDF ... both on their own, and combinations. PVA, epoxy... and I was going to do epoxy with fiberglass tape, but I think this will be too much work, like Paul noted.

  7. I've sprayed them now with primer. Best results with fibreglass reinforced auto filler, with regular bog over it for a smooth finish. Perfect result so far, but quite a bit of work. I was talking with a guy in Autobahn who does a lot of this kind of thing. He said what he found to work best in the long term was fibreglass resin over the entire box, inside and out. Lenehan use HDF and get a good result.

  8. SOS .... Where do I buy HDF ?!

  9. In that shop called Unobtainium suppliers where you can find 18mm sheets hanging from the sky hooks.


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