I just ran into a issue when aligning my door. I did a repair in my lower driver door rear. Went I was aligning my door the rear lower section was off. I tried to hammer and dolly the corner back but it damaged the filler that was on it. The entire door was flooded with filler and sanded off. Then primed and blocked. I was going to prime it another two times at least. I broke all the loose primer off. Can not apply the same filler and redo it? It was about 1/8 of a inch. It was short string filler on the bottom of that corner. Would need less filled now the angle is right. My question is the chemical bond. Filler is no more then 2 months old.
Make sure the panels are fitted before body work to avoid this problem. But yes if you sand all of the loose stuff off you can apply more filler and re-prime
I am not exactly sure what you are asking but fillers don't really chemically bond to things they bond mechanically to the sand scratches. You will need to feather the existing filler back from the edge where it has broken off so that you have an area to apply the new filler over the old filler so that you get a good bond. I would go back at least 2" and feather edge it down to the bare steel where it has come off. This should also ensure all the remaining filler is still adhered to the door. If it starts flaking off at the feather edge you need to keep taking it back until it stops doing that. You cannot just apply the new filler up to the the existing edge of the filler on the door or it will fail at that point. Make sure you give everything you are covering a good 80 grit sand scratch and proceed.
That is almost word for word what I did already. My plan is to fill the corner and then run glazing putty after to help feather it in over the part with where it meets the primer.
My only worry is how bad the rest of the filler is fractured or lost some of its "grip" on the metal. Best bet would be to take wayyy more off than you think or as mentioned above, strip the door and start over.
I took my bondo and paint stripper eastwood drum stripper to it. If it stayed on without pulling it then its not coming off. I went as far until I needed to really dig into it to get it off. My big fear was the new stuff sticking to the old stuff.
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