Paint Sanding

[cars56]

What is the grit progression for sanding over existing paint ?
The goal is to paint over existing paint without going to bare metal.
Can primer be sprayed over existing paint prior to painting?
Suggestions appreciated

[BRENT in 10-uh-C]

You gotta define "paint". Is it lacquer, acrylic enamel, synthetic enamel, catylized urethane, BC/CC, ??? Each type has a seperate requirement. Same applies with primers/sealers.

.

[ldj1002]

For the past 55 year I have painted cars. I am not a professional and the paint isn't show quality but does look good for a daily driver. Of all the ones I have painted I never knew what kind of paint was on the car before but if the paint wasn't flaking or cracking I just sanded ending with 400 and used rattle can primer only for areas where went to bare metal. Most I used was acrylic enamel. Sometime to go cheaper I use alkyd enamel which is what was probably used on cars and tractors of that time. I think my paint probably looks more like what the model came with than does the clear coat base coat used today by most.

OH, BTW Brent. I am not degrading what you said. You are a professional. Cars 56 or me could never make a car look as good as you no mater what materials used. I am a retired and you could not have done my job as good as I did just as me and other hobby painters can't do your job as good as you.

[cars56]

Thanks guys
What is a good backyard shoot paint these days? Something that does not need a supplied air respirator.
Acrylic poly I believe is on the car now.

[Lanny]

You can buy a pretty good (painter quality) air respirator at most parts stores, or at any automotive paint store, usually under $25.
Definitely USE ONE no matter what kind/type of paint you use.




.

[PetesPonies]

Sponsored Links (Register now to hide all advertisements)

If you have some specific questions about painting or body work, PM me. I'd be glad to help.

[cars56]

Thanks everyone as always!
Rob
Cars56

[PDXCougfan]

Rule of thumb i have heard from my painting buddies is always sand down to p400-p600 before applying base/sealer. Any more than that the paint will not have much to adhere to.

[cars56]

Good advise all
Final sanding willl be 400 wet, smooth but still some bite
Wet keeps the toxic dust down

[Kevin in NJ]

The proper grit is usually defined on the tech sheets for the product you are using.

Final grit is sometimes dictated by the color you are sanding. The darker the color the higher the grit. Sanding dry is sometimes wise with the correct paper as you get better control because you can see how it is cutting as you go. You can tell if you have a level surface as you go and not get surprised when the water drys. When sanding dry the finish is equivalent to a grit about 100 higher in wet. So 400 dry is like 500 wet. You need open cut paper to sand dry.

[mrtexas]

Originally Posted by cars56

Thanks guys
What is a good backyard shoot paint these days? Something that does not need a supplied air respirator.
Acrylic poly I believe is on the car now.

I'm currently painting my 29 roadster. I bought a supplied air respirator, a Pure Air 2000, very cheap $500 insurance.

http://www.eastwood.com/pure-air-200...rator-med.html

It is unique in that only one air hose is required. There is a gizmo on your belt that hose attaches to. Then one short hose goes to the gun and one short hose to the respirator. I am using a Sears 240v oil-less compressor that supplies plenty of air for an HVLP.

However while cleaning out the gun and disassembly, I use a 3M half mask respirator. That thing works good as I cannot smell any paint fumes while wearing it. I believe the 3M mask is safe.

While surfing the web I found the real danger of the epoxy/urethane iso-cyanate catalized paints is they can induce asthma if used without supplied air for long periods of time like at a paint and body shop. I decided that $500 is very cheap insurance for safety sake, why risk it even though if painting only one car?

[mrtexas]

Originally Posted by cars56

What is the grit progression for sanding over existing paint ?
The goal is to paint over existing paint without going to bare metal.
Can primer be sprayed over existing paint prior to painting?
Suggestions appreciated

I believe you can seal the existing paint with epoxy primer and then spray urethane, but why? You would then have a thicker total paint film which is not good and chips easier.

Someone who knows for sure correct me if I'm wrong on that.

[Kevin in NJ]

I talked to a university researcher about the iso paints. The isocyanates are colorless and odorless. Charcoal masks filter the iso, but it is easily absorbed though the skin too. Most of your exposure will be through the skin. So you must also cover up. I bought a hood to do painting with my pressure fed air.

Some tyvek suits from a home improvement store will do the job. Keep in mind that the 2 big sources of dirt in the paint are from the item being painted and your clothes.

[cars56]

Kevin, so wet 320 should equal 400 as a final sand, is that correct?

[PetesPonies]

No, wet 320 is not the same as 400. For a final sanding use 400 wet or 600 wet. I use 600 when spray metallics and 400 for solid colors. Wet sanding helps you clean the vehicle as you a preparing it. It keeps the paper clean and you get more done.

[Dave72dt]

Adding a few drops of dish soap to the water also helps keep the paper from clogging and the paper loses it efffectiveness after a bit. Don't be afraid to change the paper. It'll cut faster and you'll spend less time sanding, even with water.

[BRENT in 10-uh-C]

Originally Posted by PetesPonies

No, wet 320 is not the same as 400. For a final sanding use 400 wet or 600 wet. I use 600 when spray metallics and 400 for solid colors. Wet sanding helps you clean the vehicle as you a preparing it. It keeps the paper clean and you get more done.

Hey Pete, for what it's worth, I am not so sure that I totally agree with those statements as much as I kinda think it is a personal preference. I am/we are of the opinion that water sanding makes just as much of a mess and often times will allow the residue to be carried down into a seam or hidden area and when the water dries, it causes the sanding residue to stick in those places seemingly to come back and haunt you during top finish coat. Again, it is a personal preference but we dry sand everything and then use the high-pressure water side of our steam cleaner to wash out the hidden debris after blowing out with a powerful air nozzle. Then a final static blow down inside the booth with the fans running will generally dislodge most of the last debris/trash.

The other downside to wetsanding is that we can use any length of blocking paper (i.e.: 18"+) when we block dry, ...and you are limited to about 11" in wet paper. That means we are covering 1½ times the surface per stroke as what you are. We dry sand everything to 500 grit and then spray Wax & Grease Remover to check for final imperfections.

.