The Stovebolt Site Forums The Shop Area Paint and Body Shop Pinholes in Primer

I seem to get nowhere on these body panels, every time I sand I get pinholes in the primer itself, not where it has body filler. Its a high sandable primer by U-Pol. I can never finish a panel, they keep popping up. What causes this? I have yet to set up my compressor with all the water separators that I plan to use with the base coat, perhaps it's water? I dunno. Thx

Has there ever been silicone based wax used on the vehicle? Even a minuscule amount of silicone, which can penetrate all the way to the base metal, can produce the problems you're getting. Unless a panel has been stripped all the way to bare metal, silicone contamination is difficult to remove. Also, oil in your compressed air, not water, can be a huge problem. If your compressor has some piston ring wear, it can be contaminating the air you're using for your spray gun. The air filters that use a roll of toilet paper are pretty effective at catching oil from a compressor. That's what we used on the air lines to the spray booth at a couple of body shops I've managed.

McMaster-Carr catalog reference: Part numbers 9841K91, 92, or 94, depending on the pipe size of the fittings.

www.mcmaster.com/catalog/130/1237/9841K91

Jerry

Also, some of my high school students who had a habit of using "Vaseline Intensive Care" body lotion would contaminate primer when they would rub their hands over a panel they were sanding. We had a rash of "fish eyes" in finish paint jobs until I figured out what was causing the problem.
Jerry

Ok I will install one and see if it resolves the problem..not sure about any of the silicone....the weird thing is the pinholes are appearing in the second/third coat of primer which leads me to believe it may be oil like you are saying...I keep priming/sanding/guide coating/filling/priming again/fill pin holes/ fill more pin holes//etc...thanks

Here's another trick the pros use for dealing with pinholes. Spread a thin layer over the blemish and block sand.

https://www.ebay.com/itm/134683238562?

Jerry

As HRL has pointed out, pretty much any oil based or silicone based contamination will result in pin holes/fish eyes. Also, possibly it is too heavy of a coat. As long as you're shooting a sanding primer, try reducing the thickness of the coat while spraying.

Please make sure you have the right temperature reducer as well as hardener. It is made in low temp, medium temp and high temp (some makers will call this slow dry). Right now you want the slowest drying thinner and hardener they make. If your reducer is drying as you spray (and it will if you have the cool temp or quick dry type), you'll get lots of pinholes. Try to spray only in the morning when it is cool and humidity is higher. It might sound odd but your reducer can be partially dry before it hits the surface and if the surface is warm to hot (as it can be this time of year) it will cause the reducer to dry faster and the hardener to react faster and this creates air pockets. At least it does for me. The fact it is not happening where you have body filler tells me this is acting as an insulator of sorts keeping the surface area there cooler. Good luck!

Edit...if U-Pol doesn't make temperature-sensitive thinner you can add about 10% extra when you mix the primer. Just watch how you spray so it doesn't run...you might have to adjust your gun for this. Extra thinner will reduce the build-up aspect, however for primer to dry solid and without bubbles or air pockets, the thinner has to keep everything bound together and let it dry correctly. First a film coat will form on top, then the primer will bond/attach to what is beneath it and then the middle will start to harden as the thinner evaporates slowly through the film coat. Heat and air passing over it will accelerate drying. In the case of catalyzed primer the catalyst will produce a small amount of heat as it reacts, also. Not the same type of heat as fiberglass resin or epoxy but the reaction is about the same from what I've been told.

Thanks for the replies..I was putting extra reducer in it because my gun was having a hard time spraying it 1.8 tip I believe

If it is high build primer, that could be part of the issue. Hard to say, though. I usually use a very old copy of a very old Binks siphon design which I've increased from 1.8 to a shade over 2mm...maybe 2.1mm. I didn't expect the increase to make a big difference but it did. Also with that larger tip I lowered the pressure a wee bit (a shade over 25 psi) and that helped with the thick stuff...but as I mentioned earlier I have to watch how I spray now. At times in the summer I just use plain old lacquer type primer and hot temp reducer. Like this: https://5staradvantage.com/product/301/

I know...folks don't like it these days but it works in hot weather as long as I cut it 1.25 reducer to 1 primer. The red was my choice but it is discontinued.

A friend went to Harbor Fright and bought 4 of the HVLP guns they occasionally mark down to $9.99 each. He increased one to 2.5mm and the other to 2mm one to 1.8mm and one to 1.6mm (I believe they are made with a 1.4mm tip) and says they spray about as well as anything he has if he cleans them really well after using.