Underbody tray scratch repair

The most important part of doing a repair like this is surface preparation. Make sure to grind out the scratch and surrounding area to expose fresh material. Use a coarse sandpaper of 80 grit for this type of work. Do a final cleaning using denatured alcohol. Use a high quality epoxy that's flexible and overfill the crack. If it's deeper than 1/8", add a glass fiber filler to keep the patch from cracking. After setting but before fully hard, use 80 grit to sand it down to nearly level, and 120 grit or higher to finish the job. Then paint.

The important thing to keep in mind is just filling the scratch is usually not good enough. The repair should overlap the damage by at least 1/2" on all sides. That way, the repair becomes part of the tray and won't flake or delaminate.

As far as painting goes, spotting it will likely be unsatisfactory. The paint should at a minimum be blended in to the edge of the area or it will always show up as a patch.

Good luck!

 

I just done mine, find some plastic bumper repair filler, worked wonders as very fine and bonds very well, just a fine sanding pad to smooth out.

 

Not sure what that is, avoid anything that mentions fibre glass. Try this pic to see if u can find something similar. This is a 2 part filler
Image Unavailable, Please Login

 

Used regular filler on mine. No problems.

 

Ive worked with fiberglass a 100 times and also plenty of bodyputty but when i tried repairing the center undertray on my car and i had problems getting the fiberglass to bond. I believe the undertray is made of FRP fiberglass reinforced plastic.

 

Mechanical bonds work great with epoxy. The trick is getting the surface rough enough to provide the strength needed. Typically, that's 60-80 grit rough. For a plastic, I'd go with 60 and dig in deep. Once the surface is scratched deeply, fiberglass should hold just fine. For a broken piece or a crack, a repair to both sides would be required and the old material sandwiched between new material to get a good structural bond.

The bumpers are made of another weird composite called "Rangerplast" which seems to be cornstarch with a resin binder. It's very soft so nothing can bond to it directly. It also has to be sandwiched to get any strength out of the bond.

 

Exactly! I tried using the Bondo fiberglass and it will NOT sand smooth. What a mess I made!

 

I'd agree with that. I use a lot of bondo for molds, fabrications and such, it's great. The stuff with the fibers is just wrong. Why would anyone want to use bondo for a structural repair?

 

This is the finished job after mine got damaged going on a ferry Image Unavailable, Please Login

 

This thread cracks me up (in a good way). You guys are touching up scratches on your under panels that only mechanics can see, meanwhile my undertrays are the single most hammered part of my car (by a significant margin). They look like I ran over a scree field. I have repaired everything with fiberglass (followed up by black paint) and everything is structurally sound, but it looks like death, and I'm fine with that as its under the car!

 

I have to repair my driveway, not my scratched up under panel.

 

@Scottslaw - I think one of the reasons why owners are concerned about their undertray is that it shows a lot about the car. Undertrays that are dinged, scratched, and broken show the car was handled roughly by an uncaring owner. That may or may not be true but it is an important data point. A car that has a clean and virtually unblemished underside indicates the car has been well cared for, even the parts that don't show.

Look at BAT. Cars with scarred undersides go for a lot less.

 

Good point,

I will just repair the undertray --- right before I sell the car.

 

The problem is the main dealer writes it up every service, i had one condition report to say the car was not presented in a clean condition for its service, as i had driven 3 hours in pouring rain i am not surprised.

 

I was in the dealers showroom and pointed out a dint in a new one, looked like a door had hit it.

 

Try this stuff
Image Unavailable, Please Login