Wood finishing question

[dsrviola]

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Hello all,

I can finally see the light at the end of the tunnel! Cabinets are almost ready for veneering (all sides except the front baffle.) I'm planning on painting the front baffles to match the black frames on the accuton drivers that I'm using. (I'm guessing the closest finish would be an eggshell finish.) The front baffle is very complex. (see picture) I'm filling cracks/ breaks between pieces, but since it's plywood (and mare than a little complex) it's never going to be perfectly smooth. It's my understanding that you're not supposed to sand primer. I was wondering if there was some kind of hard core (thick) primer that I [i]could[i] apply and then sand until smooth. I was just planning on using black from a spray can for application. (shrug)

Any comments or suggestions greatly appreciated. Thanks!

[MJL21193]

Hi,
Primer can be sanded, that's no problem but you need to take a different approach. I would first sand the baffle as smooth as possible then seal it with something that will stop moisture absorption (I use clear polyurethane). Let this dry than lightly sand.
Fill the imperfections with automotive body filler (Bondo) and sand this smooth. There are two different ways to apply the filler - multiple passes or one pass. The one pass will heap material higher than the "scar" and you will have more sanding to bring it down flush. This would not be the best way considering the complexity of the baffle. The multiple pass way has you applying several thinner coats of filler to eventually fill the scar. This results in less sanding, and as the filler cures quickly, will not take very long.
Once you have it all perfectly smooth, then it's ready for primer.

[Helmuth]

Hi looks like a nasty accident with the accuton.


I just covert some parts with velours.

I have 3mm triplex sprayed with glue and then placed the velours. You have to watch to the direction of the cords.

I got the Idea of the tannoy of course with the black velours.




Vinyl leader would also be nice IMO.

[grimberg]

Sand it smooth, wipe it clean with a damp rag to raise the grain and sand it again. Raising the grain only needs to be done once and it is helpful before using water based finishes.
Next, get some Durham water putty at your local home center and cover all imperfections. Use a clean wet sponge to smooth the putty after it dries. For this step you really need to squeeze all the water out of the sponge and just keep it lightly humid. Spray it with sanding sealer or a mixture of PVA glue and water at 50%, whichever is more practical for you. After it dries, use a sanding sponge (also available at Home Depot or Lowes) in just one or two light passes to remove any dust or minor imperfection from the surface. At this point the surface should be really smooth and ready for priming and painting.
Using the regular and sanding sponges will help you deal with the complex surface of the baffle.

[MJL21193]

Originally posted by grimberg
...a damp rag ... using water based... water putty ... wet sponge ... the water out ... lightly humid.

Avoiding water in the first stages will cut the work in half and arrive at a better result. Wood swells when wet (even a little bit of water). This is especially true for the end grain of cut plywood.

[grimberg]

John,

I agree with you about using Bondo on flat surfaces and machine sanding it up to 400 grit, I have gotten great results with that approach. I thought about presenting the water putty option since it makes things easier than hand sanding Bondo.

[MJL21193]

Hi grimberg,
I have done a lot of work with body filler (not Bondo brand, but a better professional one from Sherwin Williams) both in this hobby and in my day to day work. It does take a lot of effort to sand but the results are usually worth it. It doesn't shrink and leaves a hard surface.
It doesn't need to be a large flat surface either, though it would be easier.
The front of this case is smoothed with body filler:



It is made from MDF.
Filling the defects gradually, with minimal sanding is the key.

Anyway, it depends on how smooth you want the final job to be.

[Helmuth]

Mann that looks great MJL

I know that when sanding plywood and painting it you wil always see the layers because the wood keep working at different humidity.

Even with MDF sanded perfect flat you always see the lines come after time
Therefore is a solution, finish it with a thin layer of mdf and sand it.

Thats the reason I dont paint but cover it with velours also vinyl leader is a possebility. Because it is robust against scratching and covers oneven surface of plywood.

[grimberg]

That is beautiful work John! For what project did you use that case?

[MJL21193]

Originally posted by Helmuth
Mann that looks great MJL

Thanks.

To prevent (not always possible) the lines showing through, as you mentioned, you need to get as much moisture out of the MDF or plywood as you can by leaving it in a warm dry place. Sealing the project inside and out will prevent moisture absorption and avoiding water based finish until it has been primed will go a long way.

Originally posted by grimberg
That is beautiful work John! For what project did you use that case?

Thanks,
That is the case holding the HTPC computer that I'm talking to you on right now.