Paint fail question?

PNW_Steve · 05-11-2020, 02:56 PM

Hey Everyone,

I am working on painting the roof so I can get the vents and solar panels installed.

I washed the bus with Dawn dish soap and rinsed thoroughly. Then wiped it down with wax and grease remover. Then sanded with 120 grit. Then washed it and wiped with grease and wax remover.

I sprayed Sher-kem reduced with hardener (1:

and xylene (3%) per manufacturer recommendation.

It went down pretty good until my arm got tired and I had trouble controlling the gun. That left me a few light spots an a couple of runs to deal with.

The problem is some areas did not take the paint properly. It was obvious when I sprayed it. The paint "retreated" leaving bare streaks and spots.

Take a look at the picture and tell me what may be going on?

Thanks.

S.

ol trunt · 05-11-2020, 03:43 PM

I see three things. First, the zebra effect means you aren't getting the necessary 50% overlap with each pass. Second, you aren't delivering enough material with the air/paint ratio you are spraying with and third the pull away is caused by using much too coarse a sand paper. Anything much rougher than 220 grit when coupled with low paint volume skips over the sanding furrows and only gets more noticeable with successive coats (probably where the runs started as well).

The spray gun you use makes a lot of difference in air/paint/thinner ratios and is very different between an HVLP gun and a siphon gun. Since vertical surfaces are hardest to paint, practice your technique on something like an old car door. When I have a big paint job to do I usually only fill my cup half full so I don't tire as quickly.
Jack

EastCoastCB · 05-11-2020, 03:56 PM

ol trunt gives some good advice.

I kinda see in the pics what you're talking about. Sounds like you maybe had some film of something left on it before spraying, idk.
I'm about to embark on a long journey to paint my bus and its daunting! Good luck Steve!

ol trunt · 05-11-2020, 04:03 PM

Good point ECCB. I've seen paint jobs where every swipe of the tack cloth shows in the finish. This usually happens when the painter has used a tack cloth intended for woodwork rather than auto finishes. The household tack cloths are VERY sticky and leave a trail of resin on the finish in their wake. Automotive tack cloths aren't very sticky at all.
Jack

PNW_Steve · 05-11-2020, 05:00 PM

You really have to zoom in to see the problem I am having. The first picture shows it best.

Any thoughts?

Again we did the following:

Wash with Dawn & w rinse thoroughly.
Wipe down with grease and wax remover
Sanded with 120
Wash with Dawn & w rinse thoroughly.
Wipe down with grease and wax remover
Made sure that everything was thoroughly dry
Sprayed the paint.

The problem is only in a few areas. Not the entire roof.

I know that I have some learning/practice with the HPLV gun. I see the light stripes and will shoot another coat paying more attention to overlap.

It's those streaks and round spots that are really giving me fits.

PNW_Steve · 05-11-2020, 05:16 PM

I made sure that all of the rags that I used to wipe it down were fresh out of the laundry.

PNW_Steve · 05-11-2020, 05:44 PM

OK..... I took a "real" camera out and took more pics...

BlackJohn · 05-11-2020, 05:50 PM

A roller should fix all that up without showing, no?

John

PNW_Steve · 05-11-2020, 05:51 PM

I need to increase air, material and fan size to correct some of the little issues but I am a bit baffled by the areas in the pics.

I need to address them but the Dawn, grease & wax remover and sandpaper have proved inadequate. I have nothing telling me that if I repeat what I have already done again that I will have a different outcome.

How to identify the problem and correct it is the focus now.

PNW_Steve · 05-11-2020, 05:55 PM

Quote:

Originally Posted by BlackJohn

A roller should fix all that up without showing, no?

John

I don't think so. I came back and sprayed more paint on a couple of spots and the liquid paint pulled back from the bad spot leaving a bare spot with a thick ring around it.

I don't know or I would not be asking. I "think" that it looks like some kind of contamination but I cannot fathom what could have withstood our prep and stayed to affect the paint.

PNW_Steve · 05-11-2020, 05:58 PM

Regarding sanding grit:

80 grit was recommended by a gent at the paint store. I thought it awfully coarse so I used 120 grit.

EastCoastCB · 05-11-2020, 06:09 PM

Quote:

Originally Posted by PNW_Steve

I need to increase air, material and fan size to correct some of the little issues but I am a bit baffled by the areas in the pics.

I need to address them but the Dawn, grease & wax remover and sandpaper have proved inadequate. I have nothing telling me that if I repeat what I have already done again that I will have a different outcome.

How to identify the problem and correct it is the focus now.

Maybe you left some residue from the dawn or grease and wax remover? That's my best guess.

banman · 05-11-2020, 06:35 PM

There's either a residue there or the underlying paint is chemically incompatible...

PNW_Steve · 05-11-2020, 06:43 PM

Quote:

Originally Posted by banman

There's either a residue there or the underlying paint is chemically incompatible...

Take a look at the "pattern". It is only on part of the roof. If there was an incompatibility with the old paint I would see it over more of the roof. Not just in a few spots.

Residue makes more sense but what managed to stick around through prep process and how do i get rid of it now?

banman · 05-11-2020, 06:51 PM

Quote:

Originally Posted by PNW_Steve

Take a look at the "pattern". It is only on part of the roof. If there was an incompatibility with the old paint I would see it over more of the roof. Not just in a few spots.

Residue makes more sense but what managed to stick around through prep process and how do i get rid of it now?

I thought about addressing that -- I'm definitely not a pro-painter...

I've seen incompatibility be very inconsistent on other mediums -- I can't tell you why, but I can tell you it happens...

Or maybe the trouble area was painted for a repair before you got the bus...

Is the white paint a primer or a top coat?
How universally compatible with all the possible schoolbus paints is it?

sproutroot · 05-11-2020, 07:09 PM

You're "walking" backwards and painting with an HVLP, correct? Looks to me like your air hose is contaminating the surface behind you as you're painting. My next best bet would be tack cloths, but the pattern doesn't look right for that.

Make sure that air hose is clean. You may want to just cover it with something for this job. They get dragged through every project you use them for, so they're usually soaked with all kinds of oils and metal dust.

bus-bro · 05-11-2020, 08:16 PM

What did you use for rinse water? Maybe if you used hard well water you left some residue from the water.

The prep method that's worked for me is a thorough scrub down with Scotch-Brite pads using a solution of water and TSP, a copious rinse and good drying.