Repair visible when heated by sun?

T

Top-Gun

I purchased a new OEM bumper cover for a project and unfortunatley it came with front plate mount holes. Not wanting the ugly holes, I decided to fill them. I believe I did everything right;

- tapered the edges of both sides of the holes
- roughed with 80 grit, cleaned multiple times with SPI water based cleaner
- buried stainless mesh into back side using plastic welder
- filled from back side using fiberflex welding rod and plastic welder (bumper is PP+EPDM)
- roughed weld material from front side, cleaned
- applied two coats of 3M epoxy flexible filler with recommended adhesion promoter
- sanded flush
- allowed filler a week to cure
- two coats of SPI epoxy
- two coats of SPI 2K Reg build
- blocked down to 600 wet
- sealed with SPI epoxy and sprayed with RM onyx base (jet black) and Glasurit clear

When the bumper came out of the booth, it looked absolultley perfect. It has been sitting in my shop for a few weeks and still looked great. Today I took it outside to fit seals/trim etc. and after an hour in hot sun, I could see the outline of the plate holes plain as day. I could feel the filled areas had swollen ever so slightly proud of the surface.

I took the bumper back inside and after a short time out of the sun it looks perfect again, no sign of the repairs whatsoever.

Any idea what's going on here?
 
I would have used a 2 part rubber bumper repair material instead of a plastic welder, they have different types from flexible to rigid. When using a plastic welder the heat can change the properties of the material being fixed causing issues also if not done properly the weld might not have fused properly to the material being fixed. The sun warms the bumper at a different rate then the repair causing it to expose itself.
 
the fiber flex is good stuff. i use it here and there however dont get the wrong idea in thinking you are doing a plastic weld. fiber flex is nothing more than a heat activated fiberglass reinforced bumper repair material. its good when you dont know what the plastic type is that you are fixing. the fiberflex is a dissimilar material from your bumper so it CAN expand and contract at different rates from the surrounding material. the best thing for you to do in this situation would be cut a small piece of plastic from the back of the bumper somewhere and then do a proper weld. when you do it this way it will be far superior than any repair material that you can use since there really is no repair. your simply melting everything back together.
 
Thanks for the replies. I never thought the differences in expansion would be quite as noticeable.

I played around with the old bumper cover and tried the pure polypropylene filler as well as the TPO filler and I just didn't like how they welded (hence me using the fiberflex on the new part). I guess I should have sacrificed some of the old bumper to use as welding material.

Do you think this is mostly a cosmetic problem or should I be worried about the filler cracking/paint failling down the road after its been cycled a few times in the sun?
 
Welcome to the wonderful world of ghost shadowing. All those steps after the fiberflex do nothing for the underlying issue. The repaired area may never crack at the holes, it'll just always show up in temperature changes. Probably cosmetic only.

I just walked in the house from doing plastic welding outside, With a hot air gun &matching donor material, as Jim mentioned. I'm guessing you have one of the little triangle shapedtools, such as harbor freight sells? I only use the contact type[soldering iron] type tool as an aid to flattening out melted edges from hot air welding--- while still melted. A pure pp ribbon strip is ok, as is any pp labeled bumper part. Your issue is not having the proper tool. In proper hot air welding, you blow hot air between the strip & the repair surface, melting the side of the 2 strips that are attached together. You don't have a complete molten area like with metal. You must have the surfaces of both parts melted to bond or it ain't gonna be right. With a plastic weld , the material doesn't have the molecular crossbonding of the original material, so extra thickness from the backsides is best. I generally run a ribbon front & back of a repair.;
You need to be careful to retain the shape of the part when welding. Thicker donor parts are harder to work with than premade ribbons. I sometimes heat on both the front & back of the strips to make them bend without pushing as hard on the part being repaired.

Good 2 part adhesives can work great , but if the part ever gets flexed hard enough , it may just let loose.

Polyvance, formerly urethane supply co, has tools supplies & info.
 
jlcustomz;n84785 said:
Welcome to the wonderful world of ghost shadowing. All those steps after the fiberflex do nothing for the underlying issue. The repaired area may never crack at the holes, it'll just always show up in temperature changes. Probably cosmetic only.

I just walked in the house from doing plastic welding outside, With a hot air gun &matching donor material, as Jim mentioned. I'm guessing you have one of the little triangle shapedtools, such as harbor freight sells? I only use the contact type[soldering iron] type tool as an aid to flattening out melted edges from hot air welding--- while still melted. A pure pp ribbon strip is ok, as is any pp labeled bumper part. Your issue is not having the proper tool. In proper hot air welding, you blow hot air between the strip & the repair surface, melting the side of the 2 strips that are attached together. You don't have a complete molten area like with metal. You must have the surfaces of both parts melted to bond or it ain't gonna be right. With a plastic weld , the material doesn't have the molecular crossbonding of the original material, so extra thickness from the backsides is best. I generally run a ribbon front & back of a repair.;
You need to be careful to retain the shape of the part when welding. Thicker donor parts are harder to work with than premade ribbons. I sometimes heat on both the front & back of the strips to make them bend without pushing as hard on the part being repaired.

Good 2 part adhesives can work great , but if the part ever gets flexed hard enough , it may just let loose.

Polyvance, formerly urethane supply co, has tools supplies & info.
Click to expand...


I find these posts really interesting because I have never really dealt with that level of plastic repair, I'm a complete hack at it since I've always had to do work for shops dealing with used car dealership stuff. They never really cared how long the repair will last or if too complicated they just plain ordered a new bumper and called it a day.

Would it be possible for you to post a picture of the tools you use jlcustomz, I would really really appreciate it?

I want to learn more about this subject and know the proper way to repair plastic.

Thanks in advance.
 
Great info there, thanks for taking the time to post that. You're correct in that Ive been trying to use the Polyvance airless plastic welder, which is basically a fancy adjustable soldering iron.

I found that trying to get good adhesion of the filler material required a tremendous amount of heat input, as you had to liquify the substrate and pump the filler in with it still gooey otherwise the filler could be picked off. I could get a good bond, but the amount of heat required typically distorted the part. If I remember correctly Polyvance actually recommended the use of Fiberflex rod rather than trying to weld with polypropylene filler.

Since this was a $2 grand + bumper (Porsche), I didn't want to risk distorting it and opted for the Fiber flex method. Sounds like the right way to go is the hot air welder and I'm wasting my time with the airless rig.

The 3M filler product I used was a two part epoxy specifically for flexible TPO bumpers, should be good stuff so I guess the Fiberflex backer must be pushing out on filler slightly when it gets hot. Suprised the mesh I melted into the backside doesn't help more.

Live and learn I guess, hopefully it stays cosmetic as I'd hate to strip/repaint this thing.
 
I don't get how it's the plastic repair materials fault if it's on the backside and the 3m filler is on the front. Sounds like a filler issue.
 
If i was doing it i would sand with 80 grit, clean with waterborne cleaner, blow dry, spray light dust coat of fusor 602 ez, 700 mat, fusor 152 and let dry 1 hr and feather out. If any imperfections on the front use fusor 114 sand with 320 and prime. Here's a video on it

https://youtu.be/n3nglBO0Tic
 
I'll post something on tools later when I get a new camera lens.


Holes can be one of the worst things to fill with a different material as far as shadowing goes, much worse than cracks.. Filling them with fiberflex is kinda like filling holes in metal with an epoxy adhesive, it'll hold , but will always show up in temperature changes. Epoxy will show holes more than polyester filler. Most of us have seen a decent looking metal panel before stripping which turned out to have old style dent puller holes filled with poly filler & the surface skimmed.



Doing welding ribbon repairs without excessive distortion takes a little practice &; technique.



Years ago someone at urethane supply told me fiberflex wasn't the most solid or stable, just more universal & easier for the average person.
 
jlcustomz said:
I'll post something on tools later when I get a new camera lens.


Holes can be one of the worst things to fill with a different material as far as shadowing goes, much worse than cracks.. Filling them with fiberflex is kinda like filling holes in metal with an epoxy adhesive, it'll hold , but will always show up in temperature changes. Epoxy will show holes more than polyester filler. Most of us have seen a decent looking metal panel before stripping which turned out to have old style dent puller holes filled with poly filler & the surface skimmed.



Doing welding ribbon repairs without excessive distortion takes a little practice &; technique.



Years ago someone at urethane supply told me fiberflex wasn't the most solid or stable, just more universal & easier for the average person.
Click to expand...
The fiberflex is on the back of the bumper. How is that going to come through to the front?
 
Stuff is mushy, not solid like the matching material welding me & you do. The thin soft filler material on the front can't hide movement underneath.
 
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