What grit to use on primer?

[Normspeed]

I'll be sanding my primer coat on my teardrop fenders and shooting some semi-gloss black on them tomorrow night. What grit paper should I use to smooth out the primer coat? I have 320 and 600.

[randroid]

Norm,

What I have read over the last year is 200# is generally accepted as a good grit for primer-to-metal, and 400# is good for the first coat of paint. I'll be able to tell you better in a month after I've done my P-15.

-Randy

[Plymouthy Adams]

Generally when blocking primer for final coat a grit of 500 is used, can go to 600 if you wish, but remember the bite is a bit less..when blocking use clean water with some liquid dish detergent, this cleans, prevents buildup on thepaper and also acts as a lubricant when blocking. Always block with the longest block you can use in the area..longer strokes versus short ones with a 3 1/2 will prevent wave often seen down the side of a vehicle.

Piant on..Paint off...primer maily ends up on the shop floor as you will soon see...

[John Mulders]

Norm,

I have used 600 and 1000 for my car, not sure what would be needed for semi gloss, would expect the same.

The person who explained the sanding to me used a opposite color of spray (rattle can) to lightly spray over the area to be sanded. Then with a block sander sand till the color is gone. Worked well for me.

John

[Plymouthy Adams]

guide coat...

[Normspeed]

This is a quick paint job on my fiberglass teardrop fenders. Strictly rattle can work. I think I'll go with the 600 to smooth the primer, there's not much area to cover and I have several sheets. Years ago when a shop did my P15, the owner let me help, and after the primer cured he had me dry sand the primer, blowing the dust off with an air hose, instead of wet sanding. Then he shot the final color. Any opinions on dry sanding vs wet sanding primer?

[Young Ed]

Last article I read on paint your own car stuff said wet sanding is needed because the water keeps debris out of the material and keeps the sanding even. Makes sense really. If part of your paper is all clogged up with material you will no longer be sanding in that area.

[Plymouthy Adams]

I prefer wet sanding with a squeege on the side..the squeege will highlight any pinhole that is not visible in dry sanding. As you wet sand you can rinse as you go..the water will produce a gloss that will simulate the very appearance your top coat is going to provide you..ripples, depressions and such is amplified. As I cannot see your gelcoat finsih on the glass fenders, I can say a lot of glass products are very rippled/wavy druing lay up...suggest you try wet sanding..if you have trouble seeing what you need or just not confortable, dry it off and proceed with a familar process. Dry sanding in my experience will result in the paper loading up...these little dabs of dried paint stuck to the paper actually will scratch your primer.

[Normspeed]

Darn, I guess I'm just being impatient. I wanted to shoot the semi gloss on there tonight so it would have a couple days to cure before I reinstall the fenders for the Sunday show. Working on it after I get home I don't think I'll have the time to wet sand, get it good and dry, then shoot on the finish coat. I'll try to be patient. No garage, so when the sun goes down, Norm's paint & body closes. My paint booth is a six man dome tent.

[Jim Yergin]

I have been advised not to wet sand primer since the primer is porous. I was told to dry sand the primer and wait for the color coat to wet sand.

Jim Yergin

[Plymouthy Adams]

Jim...don't ever go visit a body shop...they will bring tears to your eyes. I for one will never top coat a car and then see if we have errors...not cost effective...primer is porous...but only to the extend it will hot hold out LONG TERM

[Normspeed]

Thanks for all the info folks. I've been using the fenders with the original fiberglass gel coat and rough edges since I got the tear on the road years ago, so even the fresh semi gloss black will be a big improvement. I figure the semi gloss will hide a few imperfections. One of the things I like about the trailer is that it's a 1946 home built and came with lots of imperfections. Part of its rustic charm you could say.