Knifing putty causing soft paint

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This topic contains 2 replies, has 2 voices, and was last updated by  Edward Yemm 2 years, 5 months ago.

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    Edward Yemm

    Hi. I’ve used some knifing putty over some existing paintwork. I’ve done this in a few different areas on the car. I have sanded down areas and reapplied putty where needed. I’ve been able to block off these areas. They are nice and smooth. Today I started to sand down the last area. I’ve removed the excess putty and the areas looking nice and even. This time though, the old paint underneath the putty has become soft. This isn’t the first time this has happened. It happened on the same panel last year. You can feel the paint gumming up the 240 grit paper as you sand. When i press my finger into the paint i can see fingerprints in it.

    This is the process I used in this area:
    V-shaped groove sanded in a crack in body filler.
    Both wet and dry sanding.

    The cause of the crack was rectified in the past. Two spot welds between panels were broken during panel beating. Holes were drilled. Panels riveted back together. Crack didn’t get any worse over time.

    Continued sanding v-shaped groove.
    Sanded down to 3-4mm below surface.
    Filled groove with putty.
    Putty proud of body surface.
    Left to harden for almost two days.
    Wet sanded with a block.
    Putty nice and hard.
    Hard to remove but shaping nicely.
    Sand putty so its level with panel.
    Excess putty removed.
    Paint now found to be soft around edges of v-shaped groove.

    I find that its necessary to wet sand knifing putty as, if i don’t, i find it comes away in shreds and pulls paint off with it. I used the same v-grrove technique on other areas of body filler on car and as i said before, they are fine.

    So, anyone got any clue as to why this happened?



    I’ve only seen this in two instances, old lacquer or acrylic paintwork, or very fresh base/clear work.


    Edward Yemm

    This area had filler primer, primer base coat and clear coat over it. On the adjoining panel there is some underbody spray. Its waxoyl. Its kind of a rubbery oily texture. Maybe some of that mixed with the other paint during sanding and reacted with it? Or maybe the paint under the lump of putty was subjected to a chemical reaction due to the uncured chemicals in the putty? Last year, it may have been the paint softened due to the primers being freshly painted on?
    Some sort of chemical reaction taking place. I left the paint for two more days. I tried to push my finger into it and this time its hard again. Very strange. Almost like it was contaminated by paint thinner.

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