I just applies 3 coats of high build primer on the roof of my model A. A while back this topic was touched on, about using water for wet/dry sanding and cleaning.That water is not always good to use ,that even after waiting a period of time the primer can absorbed the water, when you top coat you,ve trapped the moisture underneath creating problems. I want to sand the primer now with( wet)/dry sandpaper, block sanding w.600 grit sandpaper.Is this a mistake using (water) even if I wait a period of time. ?? Gene in Mn
where is the car in the paint process? I ask, because it seems there is a way to do this that works better....start sanding with a co****r paper, and longer sanding board, then prime again and use a shorter sanding block, and finer paper, then prime again and use a pad instead of a block, and finer paper. The grit for the final sanding depends on what top coat you're using. As an example, I'll long board sand the filler with 80, then prime and dry sand with a block with 180 or 220, repeating as needed to get it straight, then final wet sand with two sheets of 400 folded over twice (6 layers) for shooting acrylic enamel. The modern urethane primers seem to be pretty safe around water, they are not so porous like the old lacquer primer. And if water can get in the pores, then it can get back out again, which some folks don't seem to realize.
Properly catalyzed, applied and cured primer won't absorb water from wet sanding period. I have never heard of a paint manufacturer discourage the use of water as a sandpaper lubricant for sanding paint or primer. Obviously a water sanded SURFACE must be completely dry before any more product is applied.
^^^ Agree, a catalyzed primer won't absorb (enuff?) water, but should be blown off with clean dry air and allowed to fully dry before re-coating.
No matter what happens it needs to be wipped with a tack rag prior to coating. I would rub it lightly with fine paper then shoot a couple of coats of color on it then wet sand and repeat.
I've heard issues of people fill primering a car and then driving it around like that for a year or two, letting get exposed to the weather and having rust issues later, but I think that's a different situation. My guess is that the fill primer will get UV damage/breakdown over time and allow moisture underneath from regular use, fill primer is not a good topcoat. But I've never had it be an issue just from wetsanding, I think you'll be fine.
Hotrodprimer's advice is valuable for me, too. I've had my '29 AA in primer for about 25 years. Life keeps getting in the way, but one of these days I'll paint the body rock moss green and actually roll down the highway in it. Some day. Really, I will. And when that day comes, I want to make sure its prepared properly for paint.
The older lacquer primer was porous. I took two panels years ago and primed both with lacquer primer surficer, one I shot sealer on and put them both outside . The one that had only primer on rusted, but the one with a coat of sealer did not rust. Now with the catalised primers I have not had a problem these. Now we have water borne primer.
sanding with 600 only makes primer smooth, not straight I prefer to dry sand for the blocking process and its a toss up on final sanding apply a guide coat start with a 180/220 on a block or stick reapply guide coat and work up to a 320 this process should get it straight the finish sanding depends on the paint used old school enamel is cool with 320 a solid base clear would like a 400-500 if I use a metallic base clear I final sand with 600 800 if its water based wet sanding has a short life, meaning dont wet sand then paint a week later you may have an adhesion issue I always paint on a fresh scratch
Thanks much for the tips everyone I put a roof insert in which was a lot of work with many flaws, welding ,a little warpage and finally now i'm getting close. Ya I know I will start with more agressive grit and work up from there. The finish top coat will be base coat /clear coat. My concern was using water toward the end of the sanding grits that the primer would absord the water when using wet /dry sandpaper and wiping with water Was a very hard job for me,did it all myself as a novice. How do you painter do this everyday and stay normal ? Gene in Mn
Water will help the paper to glide over the surface better, you can even add a little dish soap as a lubricant. There's nothing wrong with dry sanding, it just make more dust, whereas the wet sanding will make a sludge. I'm a bodyman in a production shop. Most of the painters I've worked with have dry sanded, but as a kid when I was a prepper I wet sanded. I finish my bodywork in 220- 320 before I prime. Then the painter uses 400-600 grit before paint.
When shooting old school enamel, like DuLux and Supermax and out hers the paint needs a mechanical bond I.E. something to grip into. To fine of paper like 360 or finer and I guarantee that it wil peal. I shot Alkalide Enamel for years almost every day and we always sanded with 180 open coat paper, the white sand paper, never had a pealed or sandscratch problem ever if we reduced it with lacquer thinner, or Redirect Car , or white gas. White gas was a early unleaded fuel for outboard motors and two cycle lawn mowers. The different products we thinned it with was dictated by the temperature. When it was cold we did not reduce it just put it on the hot plate and heated till it was between 160-180 degrees. Just watched for the paint to start bubbling and sprayed it on, all it took was 2 coats. And the next day it was slick. This was before all the various reducers, thinners for different temp. And let's not forget the hardners . Straight Enamel still shoots good hot. Look at all the money youll save not buying reducers and hardeners. If you try it you will notice the different sound the gun makes and you will see some fuzzy stuff in the air, after the first coat let it tack real good then put the second coat on. It will look Orange peal but it will slick out as it dries. Frank
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