How to hide joint seams before finishing...

MDF or wood?
What type of joint is it?
What type of finish are you wanting?

if there are no gaps and its basically just the line, then sanding it level is the first step. Spackle works for holes pretty well and i dont notice it on most of the stuff i have built, but bondo works better for higher gloss stuff where the seam would show up.

the password is high build primer.

this guy did a fantastic black piano finish over mdf.
http://www.htguide.com/forum/showthr...0&page=1&pp=35

i have never done a piano finish but i have looked into it
basically you will want to use as little bondo as possible then use a high build primer.

if you use a tinted primer it will make the black coverage easier.

Sandable filler primer made for invisible MDF butt joints under flat black paint on my Orion baffles/woofer frames.

Elmer's wood filler.

bondo

Is the recommendation still bondo even if the box will be covered with a veneer or laminate?

Bondo's overkill for small seams. Lots of extra work sanding. Standard elmers wood filer works fine.

What ever you use, let it dry for a week before the finishing starts.
Just about everything shrinks!

EVERYTHING shrinks with age?? Damn. IT must be awful getting old.

You don't know the half of it TS.
Besides shrinking, different fillers have different effects on the primer and paint. You need to allow all of the residual outgassing time. Even primer shrinks into scratches and pinholes. I use auto "glazing putty" for the pinholes in the primer, then prime again.
I seal MDF first with sanding sealer. Don't know, if I had better primer it may not be necessary.
Spray cans just don't have enough solids in them. I would rather not go to full automotive primers, but they will do the job. I need to look for a full body, plain old enamel primer.
Those million dollar car paint jobs? Part of it is because they let it cure for months between prep and paint. ( tells you something about the reality TV shows doing a car in a week)

tvrgeek, What brand of sealer do you recomend?

Whatever the store has. Or, You can just use thinned shellac. The old famous "Spit Coat"

If you use automotive finishes, read their docs. I can get DuPont locally, so that is what I have used. I lean that way for toughness. A friend who owned a high end restoration shop preferred lacquer primer, as you could spray and sand every hour. I think it was also so he could keep in in the gun and not have to fool with cleaning it all day.

How stable is it? I noticed it's water based, so what happens when you put water based contact cement on it, or water based (latex) paint on it? Will it soften, or what?