On Feb 14, 2:06*am, blueman wrote:
This is probably a really basic question.
But is there any general rule of what the final grade of sandpaper
should be for a project that you plan to prime and paint?

I have always sanded up to 220 but wonder whether that might actually be
"too fine" for paint to stick optimally.

For wood:

IMO, if it is an "art piece" like a jewelry box that won't get much
wear or cleaning, then go to perfectly sanded 220.

If it is something like a shop built kitchen cabinet where you will
put the normal one coat of primer, then two coats of finish, a
meticulous sanding with 180 grit will work well. Use 220 to sand
spackled and filled areas as they are prone to scratches.

Outside (as in house), the rougher the better as the paint will have
to face endless expansion and contraction. It needs all the help it
can get.

For metal:

You can't get the surfaces smooth enough for something like a metal
door. I still don't go past 220, but since metal paints go on much
thinner, you have to be even more careful when sanding as the
imperfections of the surface will come right through metal paints. No
amount of paint will hide the imperfections unless you are going to
put enough on to compound out as much as possible. Still, proper
sanding is the key.

For fiberglass:

See notes on metal.



Good sanding technique is (to me) even more important than than the
grits, paper, and machines.

But I think you brought up a good topic. Most people really over
sand.

Personally, I think it is because they are putting of finishing as
long as possible. =^)

Robert