Repairing fiberglass/smc miata hardtop?

I have a daunting task ahead of me. Repairing a heavily damaged miata hardtop for the challenge car.

My understanding is that these are actually smc, not fiberglass. Might be wrong,  blame Google. 

So, i have multiple cracks to fix, two broken off mounts. Cosmetic and structural repairs galore. Below is a sample. 

I have made fiberglass sub boxes and done bodywork (straightening bent sheetmetal, filler work, patch panels, blocking, paintwork). I am an ok shadetree guy.

Materials for repairs: im assuming epoxies and woven cloths? 

On the latches: can i start by using some 5 minute epoxy to put the pieces back together, then grind out the seams and add resin/cloth for structural purposes? 

Howabout the cracks? Hit it with the flap wheel on the grinder, lay in some resin and cloth? 

What am i looking at, and needing to learn here? Feel free to link me to articles and videos that will help. 

Yep, you're on the right path.  There's nothing too difficult or scary here.  Open up any cracks wide enough that you're filling across the gap, not just jamming material straight in.  Reinforce it :) 

The only trick is to not use standard fiberglass polyester resin, as (apparently?) it doesn't bond as well to the SMC.   I did two of 'em now with the West 105 resin with the 205 hardener, worked great.   My understanding is that SMC is basically chopped up fiberglass mat with the activator/hardener mixed in so it can be vacuum cured easier.  

The original link that everyone followed for it was here (note, original site offline, this is an Internet Archive link): https://web.archive.org/web/20201012134135/https://sites.google.com/site/yourmiata/hardtops/miata-hardtop-smc-repair

The 105 looks like standard epoxy resin. What am i missing?

And that link is awesome!

there's a lot of resources for doing bodywork on corvettes, which used SMC on body panels. I know my dad has some specific resins and fillers for it, I can ask him later what they are. But the general idea is to grind out a reasonably large area around the crack and then build it back with your repair as stated above

Following along as I have a hardtop with possibly more damage. Not in a hurry to fix it though...

Yes, I have heard they are SMC, not fiberglass, but I can't tell you what that means other than more expoensive repair materials I'm told. :)

I think you are trying to avoid polyester resins, not epoxy. But I could be 100% wrong. 

SMC has a release agent built into it so that they can press it into a mold without sticking.  What you can not use is polyester or vinyl ester resin, you need epoxy resin to get a bond.  There is some good information on composite repairs in the FAA's AC 43-13 (a good reference for a kinds of repair info).  Basically what you need to do is grind out the area in layers with the largest at the surface then layer in glass to fit the ground out areas.  You can use glass cloth or mat.

Dusterbd13-michael said:

The 105 looks like standard epoxy resin. What am i missing?

And that link is awesome!

Yep, sorry, I misspoke as mentioned below, it's the poly resin that you need to avoid.   All I knew is that the standard fiberglass stuff at the local auto parts store wasn't what I wanted :)    In my defense, the last time I ordered it was probably 8 or 10 years ago!

Awesome. 

Just ordered some epoxy resin and fresh flap wheels. 

.we'll see how this goes!

Dusterbd13-michael said:

Awesome. 

Just ordered some epoxy resin and fresh flap wheels. 

.we'll see how this goes!

Please go deep on PPE for fiberglass grinding / sanding work. I know it's not your first rodeo, but I had to say it anyway.

And if you can position a vacuum close to where you're grinding, its definitely worthwhile to maybe avoid having an inch or two of fiberglass snow on every surface in your workshop

AngryCorvair (Forum Supporter) said:

Dusterbd13-michael said:

Awesome. 

Just ordered some epoxy resin and fresh flap wheels. 

.we'll see how this goes!

Please go deep on PPE for fiberglass grinding / sanding work. I know it's not your first rodeo, but I had to say it anyway.

 

No, I need people to ride my ass about safety gear. I am the guy that was grinding on seat belts on the roof of the Corvette in the parking lot one eye shut the other one squinted instead of having safety glasses. I didn't get the nickname of Mr Safety for no reason

gixxeropa said:

And if you can position a vacuum close to where you're grinding, its definitely worthwhile to maybe avoid having an inch or two of fiberglass snow on every surface in your workshop

That's a brilliant idea that I will need to implement. Hadn't thought of that

Nothing to add, but commenting so I can follow and learn. Apparently, there's this thing called SMC...

wheelsmithy (Joe-with-an-L) said:

Nothing to add, but commenting so I can follow and learn. Apparently, there's this thing called SMC...

Lol, same here...

Wikipedia.org: Sheet Molding Compound

Last night I started by taking a carbide burr in my dremel to the stringy and fizzy parts of the ripped off latches. "Smoothed" them up, cleaned the areas, and then used 5 minute epoxy to put the latches back in place. I used the whole tube on these two latches. 

Not entirely sure how to approach the next step. I understand I need to work across the gap of the crack with resin and cloth but how do I do that without adding a lot of bulk everywhere else? 

I have my epoxy resin, woven and chopped glass cloths ready to go after work, and mounted the hardtop up to a non wrecked car so I know it's in the right shape.

In reply to Dusterbd13-michael :

grind/sand it down in a wide "v" notch around the crack, then repair with glass

In reply to gixxeropa :

That's what I keep hearing but even grinding it wide as a quarter inch how the hell am I supposed to get glass in there and Matt and resin and all that? Especially without especially without adding a lot of unneeded bulk in the surrounding areas?

In reply to Dusterbd13-michael :

I'd do the best I can to widen the V as much as possible outside of the crack, so the mat has more surface area to bond too, glass the whole area, making sure to wet it out very well and get out any air pockets, then sand it back down to the contour. I think no matter how you do it, a good bit of sanding will be necessary

gixxeropa said:

In reply to Dusterbd13-michael :

I'd do the best I can to widen the V as much as possible outside of the crack, so the mat has more surface area to bond too, glass the whole area, making sure to wet it out very well and get out any air pockets, then sand it back down to the contour. I think no matter how you do it, a good bit of sanding will be necessary

This is how I did it. 

On the two areas that had vertical cracks, I dug out a deep channel and put some aluminum strips across them as a bit of reinforcement.  

But yeah, try to dig out perpendicular areas so you can reinforce them nicely with very shallow edges.

So I'm not thinking wide enough. Got it. And I'm guessing if I grind into two parallel cracks and turn it into one that given myself a backer of some fashion and essentially adding a chunk of replacement fiberglass would be perfectly acceptable. Makes a little more sense now. We will probably revisit this discussion and alternative options when I get to the hairline cracks on the underside around the latches

Get a wide variety of cloth.  You can get some really thin cloth, cut it small and build up layers.

Also get some flock.  Mix it with the epoxy and work it into places that are too small or difficult to get cloth.  It's structural, unlike the micro glass bubbles, which are cosmetic only.

In reply to Berck :

That flock is just what the doctor ordered! Got some on the way!

the most minor cracks were around as deep as I dared, feathered out a ways, and layered with three layers of woven cloth and epoxy resin, rolled in tight with the metal rollers. 

I trim it, sand it, and check for depth and see if more mat is needed here, right? That's the next step? Trying to get a feel before moving onto these nasty berkeleyers.

Dusterbd13-michael said:

In reply to gixxeropa :

That's what I keep hearing but even grinding it wide as a quarter inch how the hell am I supposed to get glass in there and Matt and resin and all that? Especially without especially without adding a lot of unneeded bulk in the surrounding areas?

Cut the glass into strips that lay flat in the groove. 

On the more damaged areas put a sacrificial layer of glass on the inside to stabilize the area while you repair from the outside then you have the option of leaving the extra layer or grinding it off and repairing that side just like the other.