Question on wet sanding primer.

[lghtngyello03]

Hey guys, not new to the hobby, but new to the forum. Just picking the hobby back up after about 5-10 years of not doing it. I've got a question about wet sanding after priming.

Little background on my painting procedure. I've always painted with rattle cans, and have had awesome luck with them, and my finishes always came out as smooth as glass (thanks Tamiya Spray Paints, lol.) Before, I never primed my bodies to be painted, always washed away the mold release agent and let them dry. While said parts were drying, my paint cans were in the hot summer sun warming up to raise the head pressure in the can, as well as warm the paint temps up inside, so they'd flow better.

This always yielded finishes like glass for me. Upon picking the hobby back up, I found this forum around Christmas time.

I noticed that almost all of you guys prime before color and have awesome results....so I figured what the heck, I'll try to see if it will make a difference on my first build back in the hobby.

My first model back into the hobby is a Tamiya Mazda RX-7 Efini.

I've sanded down all the mold lines, scribed all the panel lines, panel washed them, and filled in some sunken areas on the spoiler with some Tamiya White putty. After that, here is my wet sanding procedure ...600 grit (for the putty areas), 800 grit, 1000 grit, 1500 grit, 2000 grit, then finally 3000 grit, and a final wash before priming.

Since it is February, and I'm using rattle cans, I warmed up some water in a bowl, and placed my Tamiya Fine Surface primer in it to raise the pressure and warm up the paint.

Shot my first light coat of primer on, let it flash for 20 minutes, then laid down my final heavy 2nd coat. The end results are amazing, and smooth as glass. I did have 2 dust particles float onto it though during this. One on the hood, and the other on the passenger door.

I wet sanded these particles out using 1500 grit followed by 2000 grit. Then, I proceeded to do as I do on a 1:1. I wet sanded the entire primed body with 2000 grit sand paper. Upon doing so, I broke through the primer in some areas, going back down to the molded red color.

Here is my question, finally after all that nonsense you read, lol.

Should I have to re-prime the car since I broke through the primer back down to the molded color?

Thanks in advance and here are some pictures of the primer before and after sanding.

Edited February 25, 2015 by lghtngyello03

[Snake45]

First off, you don't need to sand the body pre-prime finer than #800 grit, and personally I seldom go finer than #600. Anything finer under primer is just wasted effort.

Yes, you need to re-prime. If you broke through your primer with #1500, obviously your primer is very thin and another full coat won't result in too much thickness or buildup.

Personally I wet-sand primer with #800, which leaves good tooth for the color coat to grab onto.

You'll prolly get a dozen different response posts with at least 20 different recommendations and techniques. There is no "one true way," but there is likely to be one way that works best for you. "There are many paths to the top of the mountain."

[lghtngyello03]

Lol, thanks man!

Although the 600 was said to be used on the putty applied areas only before primer.

I've prepped real cars for paint before, but never have worried about breaking through the primer lol.

So the principle of a 1:1 doesn't apply? That question comes from not going with paper finer than 800 grit...the smoother the canvas prior to paint the better the end result with less chance of orange peel etc....?

[Snake45]

So the principle of a 1:1 doesn't apply? That question comes from not going with paper finer than 800 grit...the smoother the canvas prior to paint the better the end result with less chance of orange peel etc....?

Does a pencil write better on paper, or on a mirror? (Not a perfect analogy, just trying to make a point about a certain amount of "tooth" on a surface can be desirable.)

How smooth you want/need your primer for optimal results will depend on the final paint you're going to use, how you apply it, whether or not you're going to clear-coat, the phase of the moon, the price of pork belly futures, and dozens of other variables that you can only learn by experience.

And, as I implied, what works for me might not necessarily work for you. For example, "Lonewolf" and I share prolly 90-95% commonality in materials and techniques, but are very different in that last 5-10%. We both achieve great finishes. Whatever works for YOU.

[lghtngyello03]

Gotcha...I'll definitley get some plastic spoons and fiddle around with different stuff before I spray the car.

Always had great luck on 1:1 stuff with an awesome slick surface before paint.

[Ace-Garageguy]

I'm a 1:1 painter too. High-end, show quality or get somebody else. I find most everything for real cars carries over to little ones.

Every good painter has favorite techniques he's developed from years of experimentation and practice. They all have some things in common, but there are differences too. Get a good feel for your materials and their capabilities, and practice, practice, practice.

This is how I do it.

1) Scrub the body with Comet and an old toothbrush and plenty of water. Takes off the mold lubricant and provides a nice tooth for the primer to adhere to, while NOT softening details like sandpapar will.

2) Do your bodywork, flash removal, sink and parting-line corrections and primer with a SANDABLE material of choice. Sand your bodywork up to 400. Primer will fill the 400 scratches just fine. I use Duplicolor high-build over bodywork, Duplicolor non-high build sandable over virgin plastic. Shadow-mask areas you don't want primer to build up on, like molded details you want to keep.

3) Try to learn to shoot your primer slick to minimize sanding. Sand any bodywork areas as necessary, and re-prime as necessary. Give your primer plenty of time to dry. It WILL shrink, so let it get dry BEFORE you sand it. 600 grit wet or 800 grit wet is fine under the color. If you shoot your primer slick, you can simply scrub it again with Comet to give it tooth for the top-coat.

4) Shoot your color coats. Learn to shoot them slick too, again to minimize sanding of orange-peel. As MrObsessive noted, DO NOT sand metallics unless you're having to level peel, in which case you'll need to shoot another coat or 2 to even out the flake AFTER sanding.

5) Shoot your clear, at least 3 coats if you're using airbrush-thinned or rattle-can products. 3 coats is the minimum for safe colorsanding.

6) Colorsand with sucessively finer and finer foam-backed pads. I start at 2400 and end up at 12,000 grit. Use plenty of water and swill your water container out before every grit change. That keeps the bigger grit from the last time from scratching your finish as you work it with finer paper. Ask me how I know.

7) Polish by hand with something very soft, turning it often. I use 3M Perfect It Ultrafine Machine Polish and a microfiber cloth made for eyeglasses.

8) Wax or seal it if desired. I usually don't use anything after the polish because I get the look I want without it. I've heard a lot of guys talking about Future, but I'm reluctant to put floor wax on my models.

Ta Daaa...... there is extensive bodywork on this custom rear panel and where the top was chopped 4 scale inches . I used the exact techniques described above. Notice no sanding scratches show through the paint. Also, when this shot was taken, only the top of the roof and decklid had been color-sanded and polished. The paint was just that slick. You can see some fine orange peel on the back of the decklid and the front fender-door area. I've since sanded and polished this out.

Edited February 26, 2015 by Ace-Garageguy

[Guest]

I finish sanding a body with 600 before primer. Then, sand primer with 2000 before paint. Anything rougher will result in scratches showing through with Tamiya paint. It goes on really thin. I would primer the body again with a couple of coats and then sand it lightly with 2000 only. Personally, I prefer gray primer over white unless I plan on using white paint.

[lghtngyello03]

Thanks Bill...I have some of the 3m perfect it on my toolbox in thr garage, used it when I cut and buffed my car in high school.

Thanks Roger...I thought about the gray primer may shoot the next car with it. Any reason you don't like the white?

[Guest]

Thanks Roger...I thought about the gray primer may shoot the next car with it. Any reason you don't like the white?

Yeah. It's because on just about any color you shoot over it, the paint wants to pull away from sharp edges like edges of panel lines or sharp creases in the body and make the paint lighter in those areas. It's especially a problem with darker colors. I would definitely do a spoon test with both colors of primer before you commit to spraying paint on your model.

[jwrass]

Many Good points made here. I prescribe to the more tooth the better from raw styrene and body work.

Add a little Ivory dishwashing soap (Natural scent!!!! The others (Ivory brand) contain perfumes, perfumes contain oil, oil equals fish eyes) to your water and let your sand paper sit in this solution to soften the paper. The soap acts as a lubricant, makes sanding much easier and keeps the paper from loading up.

[Ace-Garageguy]

Something I forgot to mention...After having a few horrible fish-eye experiences (one of which was on a full-size (real) airplane fuselage), I've got into the habit of doing a final wipe-down just prior to shooting paint with 70% isopropyl alcohol and clean, white recycled paper towels. Don't use 90% iso, as it MAY take off some primer. The recycled (and cheap) paper towels in my area seem to have nothing on or in them (like dyes for fancy printed flowers) that will leach out and spoil your work.

If you let a model sit around for a week or two, to let the primer "shrink in" before final sanding (like I do), you'd be surprised what airborne contaminants can land on it...anything from pollen to fine droplets of household furniture polish can play merry hell with paint, causing fisheyes.

The LAST time I didn't iso-wipe a model that had been sitting before paint, I ended up stripping the whole thing and starting over. But when I saw the fisheyes forming on one side, I stopped painting and allowed the paint to flash off. Then I iso-wiped the OTHER side and painted it. ZERO FISHEYES ! Lesson learned.

Yeah, a lot of folks will tell you it's wasted effort and unnecessary. Your call.

NOTE: TEST TEST TEST. 70% isopropyl MAY HARM SOME PRIMERS. I use Duplicolor, SEM and Plasticoat, and it has no effect on them. TRY IT ON THE PRIMER YOU USE (on a spoon or scrap part) BEFORE TRYING IT ON A MODEL !

NOTE 2: If you shoot your primer really slick (NO orange peel) a thorough (I mean THOROUGH) scrub with Comet, hot water and a toothbrush (gets into fine crevices) will give you enough tooth for good paint adhesion without the risk of slightly flattening / softening details you have with sandpaper.

Edited February 27, 2015 by Ace-Garageguy

[Snake45]

I can tell you that 90% isopropyl alcohol won't take off Walmart Color Place primer, as I've stripped paint (Testor lacquer) off of it with that and the primer was completely intact.

[Art Anderson]

Some may call me a heritic, but I never use anything more coarse than 4800 grit polishing cloths to wet sand primer (I still use lacquer red oxide primer on everything!).

Reason? I want as as smooth a surface for my color coats as possible, as I am a firm believer in maximum color coverage with minimal paint (the more paint, the more surface detail I paid money to get!). This has worked for me since at least 1970--not likely I am going to change a successful method!

Art

[jwrass]

It is my experience that Isopropyl alcohol can be mixed in any proportions with water or used straight out of the bottle. I use it uncut and a final wash before I pinstripe an area or if I make a boo boo...... to wipe off a design or to clean up a fuzzy edge or to square a edge of a design

A good starting ratio is 70% iso to 30% water (that's how I mix it). I primarily use it as a static charge killer before paint more so than a cleaner although it is a excellent cleaner.

It is my opinion that if a 70/30 mix attacks your primer in any way I would use a different type of primer. As far as solvents go (in this case isopropyl alcohol) it is not a aggressive or hot solvent. If the iso/water mix is attacking your primer I would consider that a very low grade primer and would not top coat that brand with a top coat that has a hot solvent as it's reducer. Talk about opening up a can of worms..... sand scratch's, swelling, wrinkling, yada yada.

Testing before you commit is always best practice!!!!!