Yesterday afternoon I replaced some metal on a fender, ground down the welds, and applied some filler (Eastwood). I must have goofed and added too little hardner, and the filler still hasn't set up all the way. It is about 90% cured, but just won't set up fully. I have turned up the heat in the shop, and used a heat gun on it for a little while. Will this stuff eventually cure, or do I need to start over?

 

If it were me I would take it off and start over. I painted a beautiful 39 Mercury for a guy some years ago, he had done extensive customizing and body work on it. He had me come back a few years later because of a few small blisters in the paint. It was body filler that apparently didnt get mixed well and never really hardened. I had to dig out the soft spots, fill them in and repaint. Fortunately it was only 2 or 3 spots no bigger than a BB.
Anyway I think theres a very good chance it will cause problems if you leave it there. When im mixing filler I use a very scientific method. I put my filler on the "Bondo board", I forms kind of a circle, then i draw a line straight across the middle with the hardner and mix. If its hot and humid it will harden pretty fast so be ready.
My answer is probably not what you wanted to hear, but I personally wouldnt leave it on if it wont harden.
I think its more likely to expand and contract with temp changes if its not fully cured, that wouldnt do your paint any good at all.
Steve

 

I purposly mixed it light so I could have some working time, I just went a little too light........ "missed it by thiiiiis much"
What have you found is the easiest way to remove it? About 6" x 9".

 

Strip it and start over

 

Got it! Easiest way to strip it? (if there is one........)

 

40or80 grit sand paper if it's just glaze I would use 80-120

 

Paint stripper will lift it and no dusty mess and wasted sand paper. The undry filler will clog up and waste paper fast.

 

Man, that would have been handy to know. when I was doing my sail panels it had a hard time sticking to the factory lead.

 

I usually have a gallon of the aircraft stripper and I used a gallon to strip the hood I got for my 72 recently . It had 2 gallons of bondo on it from someone else . All it had was 2 dents lol. Talk about overkill lol. I brushed it on let it bubble the paint then scrapped it off then brushed more on and let if lift the filler and scrapped the lifted stuff off and brushed more on on areas that needed a second go at it. It won't lift all the material off at once you gotta do it in phases. It took me about 4 hours to do the whole hood. Once I was done I cleaned the metal with acetone then sanded the bare metal and wiped it again with acetone. So far its been sitting in primer for 2 months and no peeling or anything. I was affraid of the chemicals etching into the metal and contaminating the surface so I made sure to clean , clean , and clean as I worked my way down to metal.

 

Thanks for the tip on the stripper. Worked like a charm. It did take a couple applications on the thicker areas like you said, but it did take it off with no loss of sandpaper.........

 

That reminded me and my best friend at 14, we conned a young lady on my paper route into letting us "repair" her Nova's rusted quarter panel. We got the supplies at Sears Auto ( when you were free to buy spray paint without a driver's license) and blobbed on a pretty heavy coat of poorly mixed filler. We sat there in the driveway waiting for it to harden and waited and waited. She came out and asked us what was happening, at that point we realized we had an OH **** moment!! in our fledgling auto body business!! Sanding it did not make harder. We finally started scraping as hard as we could while she watched. When we got down to the paint we covered, she told us that she changed her mind. That lesson always stuck with us and is always good for a laugh 40 years later.

 

Body filler is so cheap . . . . . It should be fresh too. If it starts to look old I wont even use it on something important like paint repair.

It has many uses. I may use it up to a year old on something else around the shop, but not on a paint repair.