Urethane bumper...prep for painting..remove or not?
 

Looking for a little guidance here....

The stock front bumper on my 76 is ok. Actually looks pretty good..no cracks, no spiders, no waves.

Looking forward to having the car painted in the near future. Is there any special way to prep the urethane bumper for painting? Can it be sanded or must the paint be removed? Is chem stripper compatible with the urethane?

Also, I was wondering if it should be removed or a least loosened and separated from the body while being painted. If it is painted while on the car will the paint that ends up in the seam crack and chip over time?

Thanks, Joe

Have you tried to loosen the retaining nuts yet? If your car has spent most of it's life in the northeast, you probably won't be able to remove the retaining nuts without breaking the studs off the retainers or damaging the bumper cover, so there is that risk. If your cover is old, it probably won't come off in one piece. If the nuts will loosen, then you might be able to separate the cover from the body, but again, if it's the original cover it could be very brittle. One drawback from leaving the cover separated from the body while painting, is the amount of dirt you expose underneath and can't get cleaned out, as it could end up in your new paint from the air pressure passing over the gap.

In my opinion, you need to look at the age of your bumper cover and determine if it will outlast the paint job you will be getting. For example, if your new paint is expected to last 10 years, will this cover last 10 years? If it's original or replaced 20 years ago, you are on borrowed time before it starts cracking, so is now the best time for a new cover before you invest a lot in new paint?

For the best job, the bumper cover should be removed. By removing it you can clean and sand the body mounting surface for good paint adhesion and when reassembled, the gap won't look like a repaint. You can also clean the honeycomb and reinforcements for dirt control. Then you can decide whether to paint the bumper on the car (separated from the body) or separately on a stand. Which route you decide, will determine whether you use flexible paint or not.

There are strippers available for urethane at auto paint stores, but they are hit and miss as to how well they work, you have to sample it on yours to see if it will remove your paint. Regular paint strippers will deform the urethane. Transtar makes an excellent primer for urethane. If the paint on your bumper is in good condition and paintable, you can apply the Transtar primer and sand it similar to regular sanding primer, then top coat it.

Yes, if you leave the bumper cover as is, the paint in the seam could eventually crack.

Thanks for the advice. I have not tried to take if off yet. I expect it will not come off easy.

Good point...even though the bumper cover looks good now it wont last the paint job. Makes a lot of sense - sold.

But you know how it goes...start on one fix which leads to another and another....since it has to come off why not go fiberglass and seamless for a great look.....it just never ends.

Thanks, Joe

good advice from ahoover. First thing I thought of was the susceptibility of dirt and dust with the bumper removed. I will usually remove the bumper than mask the back of the car to prevent the dirt/dust problem.

Also, is the color metallic or solid? If it's metallic you may consider painting the bumper on the car, or at least at the same time in the same conditions and propped up in the position the bumper sits on the car. This is because if the bumper is lying in a different position than it sits on the car or is sprayed at a different time under conditions that are different than the metallic the paint may "lay down" differently (especially with a novice painter) and the panel will not appear to match (especially in bright sunlight). If the color is not a pearl, metallic of a very transparent color (some blues, reds yellows and some others) than you should be fine painting the bumper off the car even at a different time. Always try to paint the part positioned as it sits on the car if you can.

I painted my 85 last year. I had had the bumper off the car but elected to paint the car with the bumper on the car because I was spraying the car Admiral Blue a very metallic color. I prepped the car with the bumper off the car but sprayed it with the bumper on. I removed the bumper a few weeks ago and there was no bridging or cracking of the paint at all. I am going to be repainting the car again this year as I am not really happy with the paint job plus I'm upgrading to '96 bumpers/fenders/rub strips. When I do this I will paint all parts off the car this time except hood, doors and front bumper - too much risk damaging the paint aligning these panels while installing.

This is usually done on C3's when someone is using fiberglass bumpers. That way you are bonding fiberglass to fiberglass. You would need to be very careful bonding a urethane C4 bumper to the fiberglass rear quarter - the expansion and flex rates will be different between the two materials and that will increase the likelihood of cracking. Not saying it can't be done but it would be tricky and it's absolutely imperative that the right bonding agents and fillers be used.