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Anyone having any problems with the paint cracking on Plastic Bumpers due to flexing? Is there any special prep required for the plastic parts?



I ended up using both Rustoleum and Brightside on my flexible plastic parts ( front aeroFX spoiler and bumper and the rear aeroFX bumper and skirt ). I have had no problem with cracking at all.

The only glitch I encountered was when I initially finished painting the car and I went to take it for a drive. I backed the car into a barbeque and it scratched the paint on the rear bumper. It just scratched out the paint at the point of contact and did not spread or lift any other paint. I just sanded down that area and re-rolled it - and things were back to normal and the paint blended in with the earlier painting.

My first thought about possible cracking on the flexible body parts would be 1 of 2 things. :

a ) the paint was applied too thick or...

b ) the surface preparation of the flexible body part left the paint adhering more to itself then to the subsurface that you were painting on to.

NOW... if the paint was applied too thick ( or not thinned out enough ) then paint is paint is paint and it would tend to crack on any surface. What would be happening is that the outer skin of the paint had cured before the layer of paint between the outer cured layer of paint and the subsurface that it had been painted on to.

The outer layer would have become rigid prior to the trapped middle layer ( which is still uncured and flexible.

IF you were to really carefully look at the spider cracks... you should see one of two things :

a ) the crack is just on the outer layer of the paint skin - but inside the crack it will only go as deep as the middle layer of the paint. OR

b ) the crack extends all the way from the outer layer of the paint skin - and the crack will go all the way down to the subsurface that it was painted on to.

This visual inspection will tell you if it was too thick and cracked or if it was an adhesion problem.

IF the surface was not properly prepared then you would find the paint being attracted to itself RATHER then adhering to the subsurface that it is being painted on to.

Flexible body parts come in three types really :

a ) brand new part from the box - needs special prep to ensure that the plastic surface is fully cleansed of all lubricants used at the mold making factory to allow easy separation of the part from the mold.

b ) used car part where there is just the original factory paint job on it. This is hopefully the easiest to work on since you just have to 'scuff' that painted surface, make it dull - removing the shine, and hopefully your paint on to that surface has the best chance of adhesion.

c ) used car part where there has been various body work and paintings applied on to it prior to you coming along and trying to add yet another layer of paint. This situation puts your paint job at risk, because you are depending on all those previuos paint jobs to hold up and not fail. If they subsequently fail ( below the surface ) then they will crack and spider your layer of paint that you put on them. The answer for a flexible body part with so many multiple layers of paint is to sand down as many of those 'extra' coats of paint as you can.. trying to reduce the number of potential previous layers that might fail.

On my McLaren... the original owner had painted the car THREE TIMES. The front air dam and air intake of the front spoiler was thick with paint. So much so that even those layers of paint were cracking. I had two choices... attempt to sand down and remove those previous cracking paint jobs OR just replace the whole front plastic bumper and airdam. I just went an replaced it because even with substantial sanding I knew that I would never be able to trust those previous paintings to not fail. So by replacing with a new part from the box.. all I had to do was seriously wipe down and cleanse the new part and get the mold silicants off the part.

Don't know if this helps... but those are the initial thoughts off the top of my head on the topic.

Marq

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