Painting question

[fordfx413]

I am restoring my 3203 Autocrane for my service truck. I am at the paint stage of the project and have a few questions. I do not know much about paint so i went to my auto parts store, told them what i was doing and they gave me some paint. Ended up getting epoxy primer with catalyst and PPG single stage urethane paint/hardener. I sand blasted my crane before paint, then wire wheeled surfaces smooth. I wiped down all metal with mineral spirits. I mixed primer 1:1 and sprayed 2 coats with a 15 minute flash between. I let it dry for 2 hours. I then sprayed my paint 2 coats with a 10 minute flash between. I mixed my paint 4:1:1. Problem is its been 5 days and the paint feels soft and scratches easily. It Was 65 degrees in the shop where they were hanging but i did take a piece out individually to paint. It was 50 degrees outside that day. I figured after 24 hours the paint would be hard and could start assembling. Did i mess something up or did my paint not cure correctly? Thank you.

 

[glennster]

my best guess is that its too cold and the paint is not curing fast. see if you can get some heat on it for a few hours and see what happens. here at the body shop, we paint in a down draft spray booth. when spraying the booth temperature is set from 80 to 85 degrees F. the bake cycle we raise the temperature to 140 degrees for an hour or two to cure the paint. we can assemble and deliver the next day.

 

[dr sportster]

You need to keep the parts 70 degrees for two days --if its not dry do-over. Should be 72 degrees 50% humidity.

 

[David G]

Does the downdraft help on overspray coming back at you?

 

[glennster]

yup it draws all the overspray into our filtration system. we run 98.5% efficiency filters to meet epa requirements. the booth has a 1.5 million btu main heat stack and is positive pressure.

 

[Stephen Newell]

The paint ratio would vary from brand to brand so I assume you used the recommended formula so that wouldn't be an issue. The solvents they make for paint is different depending on the temperature you are working at. If the solvent is for warmer weather that could make it harden slow but I don't think in 24 hours it would be noticeably soft. I think you would be better off asking these questions where you bought the paint. The only thing I can think of you could have screwed up is not stirring the paint enough when you were mixing it. If the hardener isn't thoroughly stirred in it won't work.

The biggest issue I can see in your story is there was no mention of a air supplied respirator when spraying urethane. The isocyanate hardener contained in the urethane will go right through a chemical respirator and put a major hurt on you.

 

[Keith True]

You didn't by chance use the primer catalyst in the finish paint did you? I painted a set of old VW fenders a couple of years ago,they did the same thing.I really couldn'e figure it out,I had painted the body of the car a year earlier,and that had hardened up like a rock.I was starting to wonder about the age of the materials when I happened to look at the catalyst can.Same brand,same design on the can,but clearly stated it was for the epoxy primer.I had picked up the wrong can when I mixed it up.I have painted a lot of equipment at 50 degrees with urethane,the Ditzler rep told me it would cure,but the shine would suffer.It might have kicked a little slower,but it was hard in a couple of days.

 

[Rod (NH)]

You don't say which specific urethane topcoat you used. From the 4:1:1 mix ratio you stated, I assume it was OMNI MTK since the premium DCC single stage uses different ratios. If you did use MTK, there are three different hardeners and four different reducers to choose from, depending on application temperature. You are definitely on the cold end of things here and IMO SHOULD have used the fast hardener (MH167) and fast reducer (MR185). If you used the hotter versions, especially the hotter versions of hardener, that could possibly be your problem. Absent any defective materialss or mixup of additives (as Keith points out) I can see no other reason for not properly curing for that long a time. I would have expected a cure within 24 hours - or at most 48 hrs at 65 def F.