[Unsta]

I’ve got some extra holes in my rear fender from PO bolting the turn signal relay to one side, the relocated fuse box to the other, and a few other holes that don’t need to be there (one looks like the tire rubbed at some point, but no clearance issues currently)

Just wondering if anyone has had any obvious success or clear failures in filling things like this and then getting a successful topcoat on this kind of plastic and having it actually stick.

Some thoughts were:

1. Bondo Bumper Repair kit (probably overkill)

2. Some type of plasti-weld or JB weld (https://www.canadiantire.ca/en/pdp/bumper-repair-kit-0475601p.html)

3. Flex Seal paste or spray (mask the reverse side for example)

And for covering it after:

1. Flex Seal spray again?

2. Plastic specific paint

3. Truck bed liner spray

I’d like a nice flat or slightly textured look after not glossy, but I’m just concerned about product adhesion to the plastic used in the fender, or is it not “plastic” but something else?

Structurally everything is fine, but I think it’d be nice to clean this up now that it’s all exposed, I really like the idea of the flex seal the most if it’ll work as it’ll have a nice flat textured look.

Any previous experience in this area is greatly appreciated!

 

[CXPHREAK]

I modified one a lot for my thingy build.

It can be welded with the parent material cut into 'welding rod' strips using a decent power soldering iron. The catch is that you need a source for the plastic.

My initial research when doing that suggested zip ties as filler. They didn't work.

 

[Slinger]

I second that technique! I’ve had good results with using a blow torch to heat up a putty knife red hot. Then use some scrap from a plastic blue or red tarp and spread and melt it over the holes. Works great on bumpers, fenders, kayaks, canoes, I’ve done this for all kinds of plastic repair. Jb weld also has a plasticweld putty but who needs that.

 

[WingMan71]

Yes to the above ideas. ABS plastic can be "welded" just like metal. There are companies that specialize in that process and use it to fix damaged/cracked ABS body panels or put crashed bikes body panels back together.

You must have some ABS plastic to add to the piece being fixed. You can buy sheets of ABS plastic on Amazon. Just search for "ABS plastic sheets" and you'll find several to pick from.

For larger areas a hot air gun is typically used to heat the substrate and the new material being added until they soften and fuse together. Once the "weld" is done, it can be sanded or filed smooth and painted if necessary.

Google "ABS plastic welding" if you want to learn more about it. Very interesting process that I've learned how to do a bit myself.

Bear in mind that this method takes some practice to get good at. Considering the location of your area to fix, it may be overkill and your Flex Seal idea is just fine considering it is not in an area that people will see.

 

[Unsta]

Yes to the above ideas. ABS plastic can be "welded" just like metal. There are companies that specialize in that process and use it to fix damaged/cracked ABS body panels or put crashed bikes body panels back together.

You must have some ABS plastic to add to the piece being fixed. You can buy sheets of ABS plastic on Amazon. Just search for "ABS plastic sheets" and you'll find several to pick from.

For larger areas a hot air gun is typically used to heat the substrate and the new material being added until they soften and fuse together. Once the "weld" is done, it can be sanded or filed smooth and painted if necessary.

Google "ABS plastic welding" if you want to learn more about it. Very interesting process that I've learned how to do a bit myself.

Bear in mind that this method takes some practice to get good at. Considering the location of your area to fix, it may be overkill and your Flex Seal idea is just fine considering it is not in an area that people will see.

Thanks, yeah where this isn't a visible bumper or anything like that and I'm not fixing anything structural I don't have to really worry about bond strength or a "weld" style of connection. That does bring up a question though, is the fender made of ABS? I know I said "plastic" earlier but knowing so many different materials have different adhesion to paint, etc... I guess one of my questions should have also been does anyone know what the fender is made of!

If it's ABS I'd probably want to use some type of adhesion promoter on it before applying any kind of final coat (or most plastics for that matter I guess).

 

[Randall-in-Mpls]

The outer fender I believe to be HDPE. The're no adhesive that will stick to it, so it can only be heat-welded with a matching feed stock. It should hold paint with a good plastic primer.
I don't know about the inner fender. If it's ABS, it can be solvent welded using acetone. (You can make an ABS paste to fill the holes by dissolving ABS scrap in acetone.) Place a small drop of acetone on a discrete area of the plastic, and watch for softening.

 

[reclinedrelic]

Unsta you could 3D print a cover to go over that portion of the inner fender to hide the damage.

 

[Unsta]

Unsta you could 3D print a cover to go over that portion of the inner fender to hide the damage.

Where there's like 4-5 different holes in it I just wanted to seal them up, but 2 of them are somewhere visible along with the tire contact scrape. Something 3d printed would have to be quite large which I could get a friend with a larger 450x450mm printer to do, but really I'd just like to clean up what's there and get it a flat even color to have it kind of fade away in general.

The outer fender I believe to be HDPE. The're no adhesive that will stick to it, so it can only be heat-welded with a matching feed stock. It should hold paint with a good plastic primer.
I don't know about the inner fender. If it's ABS, it can be solvent welded using acetone. (You can make an ABS paste to fill the holes by dissolving ABS scrap in acetone.) Place a small drop of acetone on a discrete area of the plastic, and watch for softening.

Is the fender I'm showing here the "inner" fender as you describe?



I like the idea of the ABS weld, I can test on one of the tabs on top that will be hidden pretty easily.

Then there's attempting to paint/rubberize it!

 

[Randall-in-Mpls]

That's the inner fender.

 

[Unsta]

Ended up going with the JB weld plastic that dries black and then re-painting with a plastic compatible Rust-oleum satin black paint.



120 grit with the mouse sander, then 320 hand.


Took a few treatments of epoxy and sanding to get everything filled, but came out decent.



Multiple coats of paint, light sanding for consistency.



Would it have been easier to just buy a new metal fender and mount it? Probably... 😄 But it wouldn't be vintage!

 

[Randall-in-Mpls]

Great results! I might need to try that as a filler on the HDPE outer fender.

 

[Sidecar Bob]

I guess we'll know if it was successful in 5 or 6 years ;-)

 

[Unsta]

Looks like there's supposed to be a rubber boot in the battery tray to receive that fender as well, long gone. I thought that was kind of a crappy mounting system as-is.

Honda Motorcycle 1982 OEM Parts Diagram for REAR FENDER | Partzilla.com

Buy OEM Parts for Honda Motorcycle 1982 REAR FENDER Diagram

www.partzilla.com

RUBBER, FENDER CUSHION
80115-415-000

I bought some TPU planning on having to 3D print some flexible parts for this build (grommet, cable guide, etc) so here's the first go!

Sizing might not be quite on, I did take some measurements with a caliper but measuring vs actual fit never goes well for me first go... but 6g of filament and an hour wait isn't a problem.

 

[Unsta]

This version works great, but going to re-do it a bit longer and closed off like the OEM one.

Not sure if the OEM one is angled or not, but I did this one at 10 degrees.

Super happy with the results and fit though.



I’ll post it on Thingiverse when it’s all done.

3D design Honda CX500 - Fender Cushion Replica - REF: 80115-415-000 | Tinkercad

3D design Honda CX500 - Fender Cushion Replica - REF: 80115-415-000 created by Chris Collins with Tinkercad

www.tinkercad.com

 

[beast]

superglue and talc, put some glue on the spoty, mix in the talc and itll harden right up, sand it down and spot putty it to finish

 

[Sidecar Bob]

I think the way Unsta did it is better. For filling plastic you need something that is at least as flexible as what you are patching. If it is less flexible (like regular epoxy or super glue and your powder of choice) it can let go when the plastic flexes. The inner fender is pretty flexible...

 

[beast]

if it needs to be flexible... i recently tried an experiment repairing the body of an old bmx e-scooter for my grandson, i carefully spotglued some fiberglass screen to where a large chunk of body was gone, thn i covered it with hot glue from one of those hot glue guns. once i had it all filled in and built up nice i packed in in snow so it was good n stiff then i used rasp and file on it to smooth it down. finished to outside with black plastic roof cement and painted it up, not perfect but its holding good. i didnt really care how it looked as much as that it wasnt an open hole for the kid to get hurt on, but i imagine if someone took the time...

 

[Sidecar Bob]

Hot glue is pretty flexible so that may work, but if you are putting a lot of effort into building a bike you would want something more permanent.

 

[beast]

theres one other trick i like to do, but it only works if youre fixing the right type of plastic.
if your broken part is type 2 or HDPE you can rebuild it with a paintstripper gun and any other type 2 recyclable plastic. common type items are milk jugs, plastic oil bottles, laundry detergent bottles, plastic shopping bags and some of those little blue swimming pools. HDPE does not liquify, it just gels and then burns, so you have to be careful with it. its also incredibly sticky, if you want to smooth it with puddy knife you have to dip the knife in oil each time before you touch it. it will still stick, but not s bad. once this melted plastic cool you can sand it down and paint it. perfect match hole repair

 

[Sidecar Bob]

Unless you use the same plastic as the part you are repairing it probably won't bond very well.

Why are we talking about this when the job is already finished?