Bumper beading water after being sanded

O

old soul

Member
I have not painted an aftermarket bumper in a long time and within 5 minutes of starting to sand this one I am having a problem. It is a YCC brand (Taiwan, of course) that the customer
supplied. There is a sticker on the inside that states that adhesion promoter and solvent based primer has been applied and to sand it with 800 grit sandpaper. I figured I better follow the
manufacturer instructions. First I washed the bumper with warm, soapy water (Dawn) and then began to sand it with 800. With very little sanding done I noticed the water was
beading on the bumper where I had just sanded. Obviously I had already sanded through the primer and was down to plastic. It is a spot about 3 in. wide/ 6 in. long at the bottom of
the bumper. I have scuffed it with a gray scotchbrite with Presta paste, used water-borne and solvent wax/grease remover, sanded with 600, 400, and even 320 and it still beads water
like it was just waxed. I do not know what else to do. Please help me out collision guys. Did a lot of rubber bumpers in the nineties (body shop) but now everything I do has metal or chrome
so I am kind of lost on the current procedures. Thanks, guys.
(It is marked PP on the inside)
 
As long as you scrub it real good with a scuff pad and Dawn or sanding paste
you are good to go. Just make sure you use a good adhesion promoter
over any bare plastic.
I like to put a sealer coat of epoxy over that before base.
The epoxy really increases adhesion and reduces road rash a lot.
If you're still not sure after the epoxy, give it the old test,
put a piece of duct tape on it after 24 hours of cure, press it on real good
then yank it off. If you don't pull the epoxy off you're good to go.
 
To add to the problem, much of the time the cheap primer on those covers starts to melt when you hit it with #700. If you are very meticulous with your washing and drying you can shoot it just like that as long as you use adhesion promoter. 800, gray scotchbrite and some scuff pastes will all often leave raised scratches in the plastic that will have to be nibbed down once you have sealer on there.

We have been using gold scotchbrites and a special fine scuff paste for bare plastic. Those A/M covers we usually barely scuff dry with 600 to find any dirt or chunks, then do them better with dry gray scotch and fine sanding sponges. They will not tolerate much sanding, the goal is to degloss them only. We have to do them dry to be able to see better what has been done. If you go over an area twice, it will go through to the plastic.
 
Thanks for the replies jc and crash. Before I saw any replies I went ahead and scuffed it with a gray scotchbrite and paste all over and washed with Dawn. The water still beaded on that one spot
but was fine everywhere else. Hope that spot does not bite me later. I checked the replies and then scuffed the other bumper (a Tong Yang brand) with a dry gray scotchbrite. It de-glossed nicely
and washed fine with no beading. I plan to use ad/pro and seal with epoxy...SPI of course.

(I don't like rubber bumpers)
 
You don't need adpro where there is primer. Only on the bare areas. Adpro on primer reduces adhesion, believe it or not.
 
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