How Fine is Fine Enough?

E

Ed in Maine

Well-known member
I have just installed some patch panels and am spreading body filler over the weld seams. After the rough sanding has been done, how fine should the sand paper be so that there will be no scratches showing through the primer from the body filler?

Also, after all the sanding has been done, what should I use to make the final wipe down prior to applying the primer?

Thanks, Ed
 
in my opinion , 220 would be good enough before primer , then prime and when it has dried enough to be sanded . block it well with 400 grit wetsanding paper and reprime once again.
 
I use 400 grit, it takes less primer and sanding to get it ready for the final paint, any thing finer will not let the paint have anything to grip to
 
A couple of thoughts here....
Using steel wool probably is not the most prudent thing as the tiny shards of steel can break off or bury themselves in the substrate. If someone is looking for a way to check for imperfections, use Wax & Grease remover in a spray gun (-or trigger bottle) and spray over the surface you are checking.

Be cautious on how fine you make the substrates as they need to have a mechanical bond in addition to a chemical bond. 400 wet or 500 dry is about what most paint manufacturers are going to recommend however look on their product sheet for specific recommendations.

Ed, most high-build primers are easier to level than a filler (-like Rage Extreme) so if you use 36 grit to do the rough-in, followed by a skim coat again blocked with 80grit and then 180, you can blow it clean with compressed air and then use a good quality high-build primer such as Slick- Sand (which is a polyester primer). We use the Slick-Sand thru about 220 grit or possibly 280 and then we switch to a urethane primer such as MP282. We generally block sand the sheetmetal 5 times to achieve the results we are after. For us, we hammer & dolly, along with a shrinking discs to remove the dents first. Hope this helps.
 
Some good tips from Brent.
I would add:
1) when you blow clean use a filter ball ahead of your air tip to trap the fine oil mist from the compressor. Cheap to do and you will save yourself some fish-eye later
2) Slick Sand is great stuff, we use it all the time, but caution it has a very limited pot life esp now in summer. Don't dilly dally with the shoot once you make the mix. And clean the gun asap
3) for those that missed it spraying on a light coat of wax and grease remover gives the surface a wet look which shows highs and lows better. Circle them with a pencil, NEVER a magic marker. The magic marker WILL bleed thru eventually and ruin everything. You can also very lightly dust on some flat black rattle can which dries quickly. Then continue light blocking and you are using the black as a so-called 'reveal' coat to show highs and lows

unless you are very skilled, proper body work is very tedious, but worth the effort.

here we treat all metal to be painted, especially after welding, with 'metal prep' which is a phosphoric acid solution (same as what is in Coca-Cola).
Ospho is one brand, also DuPont makes it
DuPont™ CF-5717. I am sure there are others.
It is the best possible way to prepare metal for paint. It is an acid etch.
If you use this, DO NOT wash it off. Apply with hand spray bottle, wipe excess, then air dry in the sun is great for this for about an hr, longer if colder season. You will end up with a sort of battleship grey, almost galvanized look, which is what you want. I have mentioned this before but usually there is a know-it-all who gets on here and says why this is such a bad thing to do. Been doing this for 25 yrs; have never had a paint failure. My DeLuxe Touring looks like I painted it yesterday, not 25 yrs ago
 
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The OP is asking how fine the filler should be before the primer.

150 dry is good .

I would not use any prepsol over filler, I would think it would soak in .

I hate the stuff.

I maybe have had fisheyes in 3 jobs in 40 years, I have seen no end of problems with dudes overusing prepsol.

Just sand cleanly with no grubby fingers on the job .
 
pooch said:
The OP is asking how fine the filler should be before the primer. 150 dry is good .

I would not use any prepsol over filler, I would think it would soak in .
Click to expand...

Actually, 150 grit is too rough unless someone is going to use a glazing compound (i.e.: Icing) to help feather out the edges. 180 is better and should be the finished grit.

Prep-sol is a trade name for Wax & Grease remover, ...and if someone is using a good quality product, it really isn't going to "soak in" to a 2k filler or primer. In actuality it will just evaporate off. Honestly I can not think of a single reason why using a Wax & Grease remover would create an issue unless it was contaminated after being opened. Maybe you could expound on why you feel the W & G remover was the specific culprit?
 
Any Acid etch is a potential problem. They must be used with extreme caution.

Any acid left at the surface will prevent epoxy or polyester from curing at the metal to material interface.

Anytime you see peeling down to bare metal on a car ask what they used on the metal. I have seen a lot of cars peeling and exposing the etching primer.
 
Actually, 150 grit is too rough unless someone is going to use a glazing compound (i.e.: Icing) to help feather out the edges. 180 is better and should be the finished grit.

Prep-sol is a trade name for Wax & Grease remover, ...and if someone is using a good quality product, it really isn't going to "soak in" to a 2k filler or primer. In actuality it will just evaporate off. Honestly I can not think of a single reason why using a Wax & Grease remover would create an issue unless it was contaminated after being opened. Maybe you could expound on why you feel the W & G remover was the specific culprit?


Some people can learn things in 40 seconds, others take 40 years....
 
"Any acid left at the surface...."

There is no phosphoric acid left at the surface. It reacts with the steel to form Iron Phosphate which gives that grey color, and is inert. Which is what the whole process is about. This is called passivation and is well documented in the Metals Handbook, and has been around since at least the '40s

As I said I have not had a paint failure in 25 yrs and we paint a lot of cars

I think the problems you are referring to are caused by those who use muriatic acid (which is Hydrochloric acid). This is hardly the type of acid to use but there are those who do it anyway. Let the user beware.

Muriatic belongs only in the truck of the mason
 
Talking about finishing body filler. I like to finish my filler with 180 grit sandpaper Wash it off with Lacquer thinners to draw any air pockets or excessive die from the cream hardener. Then on to prep. Two Coats of Epoxy primer followed by two coats of High build primer and let this dry for 10 to 14 days. All Polyester products have a full cure time of 10 days or you will get shrinkage. Now block sand the high build with 180 to level things out and follow with 240 grit to finish. Then apply two coats of high build and block with 400, 600 and 800 to finish. If doing a show car block this second coat with 240 and 320 and then two more coats of high build followed by 10 days dry time and then finish sand with 400, 600, 800 and 1000 grit wet sandpaper. Now your ready for Top-coat.

The Old Tinbasher
 
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