? about Sanding scratches showing through clearcoat

[BretB]

I was looking at a friends car that he painted a few months ago and I could see sanding marks underneath the clear. The car is black. I am restoring a '69 GTO and it will be black and after it is painted I don't want to see anything like that in the paint. How do you avoid this and what grit of sandpaper would be recommended? I'm doing the metal work right now and the car will be ready for primer soon. The paint on this car has to be flawless. Thanks for any help.

[Mike K]

You can fill 3 pages on sanding but,
Final sand with 600-800. Dry or wet,your call. Dry seems to show the scratch better than wet but wet is easier.
Use a gray scotch pad double stacked over the entire vehicle to remove any imperfections left.
Use a grease & wax remover to 'wet' an area,it will show you how well you've sanded.
Shoot 2 coats of the black base and color sand it then another 2 coats of black. That should fill any scratchs left provided you sand correctly.
This is sanding in a nutshell.

[PsychoGraphic]

3M makes a real cool dry guide coat it's like a pounce bag, but it's black dust
on a little foam pad you rub all over the primer and then sand off, it's cool because it doesn't spread out where you just sanded and you can wet sand it too... it's alittle more precise than the rattle can SEM guide coat,

[KewlKustomPaint]

well nobody gave you the real thing to do.problem might have been
primer or bodywork shrinkage.thats usually the problem when you see
scratches in the clear later.Make sure your bodywork and primer are aged
well before sanding for final paint.I like wetsanding with 600 for paint.
800 is fine for sanding clearcoat but primer is best sanded with 600.
you can use 320 or 400 if you seal it before you base/clear.Wetsanding
workes the best and cleans the paper from sanding residue build up
like when you sand dry.Make sure your primer has aged as well as the bondo
before you prime it.This way it will shrink up,before you apply the paint.
Guide coat is great ,not sure if that 3M stuff holds up well with water.
Hope this clarifies what usaully happens when you see scrathes like that.
-Jeff

[DocCyber]

yea what Jeff said.......here are the principles you have to over come..swelling and shrinkage......and not only during the job but after.

here is what happens.......every layer has a density factor....and these density factors swell and shrink at differnt times.

if you have scratches they swell and shrink..if you have tape edges they swell and shrink.......the biggest culpret is the sun......the sun heats the metal the metal gets hot all the material on top of that metal heats up and swells then it cools and shrinks........thats where the denstity levels start to shrink and swell differntly.....and bingo........ghost scrathces tape lines and imperfection start showing up............the cure.

ok here is the million dollar cure.....poly.....do all your body work and then lock it all down with polyester primer again using the sandwich...
(remember the sandwich.......metal.poly..bondo..poly.....thats my hell layer locked and sealed forever)....block out the poly with 220 guide coat with 400 use a urethane fill primer and block with 600.........now you have all those odd density layers locked down with the toughest density out there .....poly........then you layer with a urethane primer/filler which brings the density back to a urethane stage....and then do your top coats.

this is an old lacquer secret back in the day when we used etching thinners........in those days we had to deal with thinners soaking to the primer and melting each layer one by one into one thick layer.....you think this new urothane paint swells and shrinks...yoiu boyz havnt seen anything till youve done a show car with 70 coats of lacquer...multi striped.....graphics murals.......the shrinkage made mosr men cry.

but it starts by understanding density/swelling/shrinkage/heat/cold

again we all have differnt approaches...this is for the new guyz....this one is a total cure..eliminates every problem and lets you sleep good at night.........and has been my fundimental foundatiuon since 1974.........again.......this is for those who are looking for a path, im not saying other paths are wrong

[Pippen]

Another factor than 'shrinkage' is 'SINKAGE'. I owned a 10 full time ,6 part time employee, successful bodyshop in Irelandfor 16 years. I have noticed quite a few differences in bodywork techniques between here & across the big pond.

One major difference is that very few of the 'quality' bodyshops ever have water touch the car at any stage during the prep work, other than a rinse with clean water right before painting. All flatting (sanding) is done dry.

I sand the filler with P40 or P60 grade, apply another very light skim of quality filler. block sand with P80 & then P180. Rub with the 3m guidecoat that physchographic mentioned & then machine sand with P320.
The substrate is now ready for a quality 2k high build filler primer. After applying the last coat of primer & before cleaning the gun, add a small amount of dark paint to the pot, recuce this in a 10:1 ratio with reducer & spray this over the repair area. This is your guidecoat for rubbing the primer. Flat the primer with P320 on a block & when you have a desirable surface sand the p320 scratches out with P400 or P500 by machine. The surface is now ready for paint.
Good luck.

[BretB]

Thanks for the help guys. Lots of good info here. What type/types of primer and bondo would you recommend?

[willz0072002]

Good call guys, but I also think materials plays a big factor. I used to have alot of shinkage, but changed through trial and error and now have a system that seems to work, and I can prime, base and clear within 48 hours without any worry of shrinkage. I have alot of luck with upol doliphin glaze for finishing larger filler work, then use transstrar 2k primer/sealer...layed on extra thick, then wetsanded with 400...sometimes 600 if shooting metallic. Also, of course temperature plays a huge factor, and in the winter I let things always sit overnight especially filler.

[Burton]

Being the n00b I am I'd like to request something..... everyone has mentioned a to let the product age and cure. Can we get some times of an average cure? possibly the product used, temps, and cure times?

When I do my body work I am usually not in a rush so the filler will sit for a few days before priming and the primer (HoK) usually sits for 24 hours before sanding and 24 hours after sanding.

[Mike K]

The 3M dry guide works great wet BUT, you have to apply it BEFORE the water. It will not wash off. It's THE BEST guide you can use IMO. It will show you scratchs that regular spray on won't even get into.
Burton, Thats a loaded question bro.:evilg:
TOO many variables are involved to give anything "exact".
24 hr. with 1 or 2 coats of primer is fine. It also depends on how much gets hammered on with these coats. "IF" applied correctly,you will be good to go.
Someone has a 2.2 tip and is laying pavement,it could take several days to fully cure.
Using some crap product and who knows.
Some turbo primers are fully cured in 3 hrs.
Just like any product,Follow the Tech sheet on the product and you should be relatively safe.
I'm no noob but I don't know it ALL either.
Time is my friend in paint work so I judge my "wait" on how I applied the product and the ambient conditions I'm working in.
Hot weather is quicker and cold just sucks.:lol:
I was not trying to be short on the poster but there are several good posts on "sanding" here and most ANY paint site,a little search time is necessary somedays.:tup:
I spend more time reading searchs than most of the current postings.

[sik_kreations]

yea what Jeff said.......here are the principles you have to over come..swelling and shrinkage......and not only during the job but after.

here is what happens.......every layer has a density factor....and these density factors swell and shrink at differnt times.

if you have scratches they swell and shrink..if you have tape edges they swell and shrink.......the biggest culpret is the sun......the sun heats the metal the metal gets hot all the material on top of that metal heats up and swells then it cools and shrinks........thats where the denstity levels start to shrink and swell differntly.....and bingo........ghost scrathces tape lines and imperfection start showing up............the cure.

ok here is the million dollar cure.....poly.....do all your body work and then lock it all down with polyester primer again using the sandwich...
(remember the sandwich.......metal.poly..bondo..poly.....thats my hell layer locked and sealed forever)....block out the poly with 220 guide coat with 400 use a urethane fill primer and block with 600.........now you have all those odd density layers locked down with the toughest density out there .....poly........then you layer with a urethane primer/filler which brings the density back to a urethane stage....and then do your top coats.

this is an old lacquer secret back in the day when we used etching thinners........in those days we had to deal with thinners soaking to the primer and melting each layer one by one into one thick layer.....you think this new urothane paint swells and shrinks...yoiu boyz havnt seen anything till youve done a show car with 70 coats of lacquer...multi striped.....graphics murals.......the shrinkage made mosr men cry.

but it starts by understanding density/swelling/shrinkage/heat/cold

again we all have differnt approaches...this is for the new guyz....this one is a total cure..eliminates every problem and lets you sleep good at night.........and has been my fundimental foundatiuon since 1974.........again.......this is for those who are looking for a path, im not saying other paths are wrong


what brand of poly primer do use?
and are using it on bare metal?