Miata Hardtop SMC Repair
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My son and I had been using my nice black hardtop on the Rallycross Miata. We wanted one that was not as nice since there is a lot of mud and dust flying around during a rallycross. So, we bought a damaged hardtop to put on the Rallycross Miata.
This hardtop had been on a car when it was involved in a wreck. The rescue team used a crowbar to remove the top and then turned it upside down onto the pavement. The top was damaged pretty bad.
The driver's side rear bracket was completely broken off and missing. There were numerous broken out sections along the rear edge created by prying with the crowbar. The passenger's side had deep scratches from scraping against the pavement.
I first attempted to purchase the missing catch parts. Many of the parts are still available, but the part that is mounted into the hardtop is not available. the catch is molded into the rear edge of the top with two screws that were used to hold it in place as the top was made.
After close inspection of the catch that was still attached and the mount location of the missing one, I decided to make a tab out of 1/4" aluminum strap and install it during the composite repairs.
Now I started researching the composite material the top is made of. I found on Miata.net that it is constructed of SMC or Sheet Molded Composite. Basically, that is a glass mat that is impregnated with the resin and release agent then pressed between two molds and heated to cure the part before extraction. Since the release agent is included in the resin, repair is more difficult since most products will not adhere. Polyester resin will not work.
So, I started looking for repair information for SMC. There are several good websites that describe the process. The most important points are:
1. Use the correct repair materials
2. Do not apply cleaner or any liquid to the open composite to prevent wicking
3. Grind the repair area to leave the most exposure for repair adhesion
Repair materials are expensive. All the repair resins I found are epoxy based materials and use fiberglass mat. The resins come in a twin tube package that uses mixing tips and a special dispensing gun. The cost was going to be about $150 to purchase the resin, a few mixing tips and the gun. I was not sure that one tube of resin was going to repair all the sections I needed to repair.
I have made epoxy fiberglass molds and parts for my R/C planes so I already had some West Systems Epoxy resin on hand. I called West Systems and asked them if their product would work for making the repairs on SMC. They did not have any experience with SMC, but did feel their product should be a good match for repairing it.
I decided to give it a try and see how well it works. Worst case is that I have to do it again later. Cost would be nothing since I already have the materials.
The first order of business was to remove all the rubber seals and grind the broken areas to prepare them for resin.
I used a Dremmel tool with a cutting disc and a diamond tipped tapered arbor to get into the tight areas. I exposed as much area as I could for each crack and break.
I then made a bracket from 1/4" x 1-1/2" aluminum strapping that came from a hardware store. I used the what was left of the mount pocket to get an idea of how to bend the aluminum and shaped it in my vise. Once it fit I attached the top to the car and made sure it fit properly. To hold it in the correct orientation, I drilled two holes from the outside, tapped the holes and installed two screws to keep the bracket secure while It was glued in. I also roughed up the aluminum's surface where it would be glued into the top.
With the top upside down, I used blue painters tape on the outside of the top to cover the missing areas so the resin would not fall out. I also used some micro-filler in the resin to thicken it and help keep it in place. The fiberglass I used was some 1" woven seam tape that I chopped up with scissors to create lose strands to fill the small pockets of broken areas. I then covered the areas with several layers of the tape for strength.
I coated the threaded holes in the bracket and the screws with some wax so I could remove them from the outside later. I put some of the resin in the mount area, then inserted the tab and screwed it in. Next I placed the chopped strands at the edges for filler and covered that with resin. Then I used the 1" fiberglass tape over the chopped glass. I had to use a clamp to hold one section in place but most of the rest was just covering with the tape and resin.
I also used chopped glass to repair the chipped front corner and all the other missing sections.
This is how the repairs looked before glassing the top side.
I flipped the top over and did the same process on the outside sections. However I only used the chopped glass on the outside work so they would still have strength but sand smooth. I removed the screws from the tab and drilled through the new glass below then installed the screws from the backside.
The repaired sections look pretty rough, but will be shaped and flattened with the angle grinder and orbital sander to match the rest of the top. This is how the repairs looked once ground and sanded to shape.
I sanded out the deep scratches and feathered them back. I also put some epoxy and chopped glass on the deepest gouges.
I used spot filler to fill any imperfections and sanded it flat.
I used some 2K primer to cover the top for flattening the finish prior to paint. It took two coats of primer to fill most of the scratched areas.
I then followed the same process I used for painting the other hardtops. I also used the left over base coat and clear coat from the quarter panel repair I did on the VVT car. I barely had enough color but it covered well enough.
Since the paint was applied outdoors it did not come out perfect. I also did not get the gun properly adjusted at first so some of the paint developed tiny bubbles since it was too thick when it was applied. I let it dry for an hour and just touched the bubbles with some 400 grit paper to level them.
The clear coat covered most of the existing bubbles but there were some in it too. I applied three coats of clear since I knew I would be sanding and buffing to finish it up.
Here you can see some of the small bubbles and imperfections.
The top looks good for and outdoor paint job. The replaced bracket fits well. I installed a bolt and used a collar under it. I have since welded a washer to the bolt for easy hand removal and painted it black. I also fixed all four latches to ensure they stay latched during Rallycross runs.
Now we have a top we don't have to worry about getting messed up.
I will post a review of how well the West Systems Epoxy holds up. I really hope it works!
As of 12/28/2014 the hardtop repairs have worked just fine. After a full season of Rallycrosses no cracks have developed or paint has chipped. Since the car gets flexed so often in a Rallycross run and no cracks have developed, I would say the West Systems Epoxy is a good material for SMC repair.