65ZR1

I posted this in paint and body but wanted to try here also.

I've been learning as I go with the bodywork on my 65 coupe. I've been using the Bondo brand resin with great results but the tacky surface added a lot of time when sanding. So I bought some waxed resin from a local Fiberglass shop. My thinking was its fresher, a lot cheaper and the wax would make finishing much easier. At first I had some issues with it hardening. It required more mekp then the Bondo brand. After a few test batches I got that sorted out. Now the issue is when the area has hardened it has what appears to be large air bubbles. I tap the air out while working with the brush and finish up with a roller. This resin is much thinner then the Bondo brand. It's almost like the resin is to thin and not sticky enough to hold everything together until it dries. Thus the air bubbles. Any advice?

Thanks,
Kris

Roger Walling

If it turns brownish with air bubbles you are using too much hardener.

Make sure the resin is for SMC fiberglass.

When fiberglass isn't fiberglass;
The DIY needs to know that most 'fiberglass' found on automobiles in roughly the last 15 years or so is not fiberglass but SMC or Sheet Molded Compound.

SMC can NOT be repaired with fiberglass resin or automotive body fillers primarily because the expansion and contraction of SMC when it heats up in the sunlight is much greater than fiberglass. SMC resin is basically glorified/higher quailty fiberglass resin but it must be repaired differently. If you repair a hole in SMC with a non-SMC compatible filler or resin the repair will butthole (assuming the repair feathers out in the first place) around the perimeter of the repair typically the first day it sees 110 degree heat radiating off the Wal-Mart parking lot and its much more likely and noticeable on darker colors. The funny thing is once the sun goes down or the car is pulled into the garage the butthole can totally go away until it see sunlight and heat again.

An easy way to tell SMC from fiberglass is SMC will be smooth on the backside of the panel while fiberglass you can feel the raised chopped glass.

Remember too on an SMC repair whether you use SMC resin or SMC filler you want to have a nice shallow 20 degree bevel (50 grit 3" grinder will do the trick) along the entire top-side perimeter of the repair for the resin/filler to not only bite into but to expand and contract with.

I have good luck with Fiberglass Evercoat SMC fillers and resins along with UPOL's SMC filler which sands very easy and was priced right.

65ZR1

Thanks for the info Roger. This is a 1965 coupe so the glass isn't SMC. The resin is turning brown but I assume I'm using the correct amount of hardener since it just begins to kick in the container after 20 to 25 minutes. Is this correct?

DansYellow66

I'm not familiar with waxed resin but if it functions like the wax fillers in gel coat I would not want to use it. That wax will gum up a bunch of sandpaper to get down to where it will sand properly. I would get a good quality polyester resin from someplace like Evercoat or Ecklers.

A working time of 20 to 25 minutes does sound within reason. It will vary with ambient temperature.

Bill32

Used to be:

Regular resin would stay sticky on top so you could easily add additional layers. Sometimes called laminating resin.
The waxed resin would dry hard on top for easier sanding. Sometimes called finishing resin.

Today:

Surprised you could find waxed.

The Bondo brand resin (sold at HomeD) is OK. It's not made by Bondo, a few years ago 3M bought Bondo. The resin still has the same 3M part number on the can but is labeled "Bondo".

If your mix is not curing on top, it's mixed wrong. Or the ambient temperature may be too low.

Air bubbles must be worked out to the edge of the mat not just broken up.

BTW, the only think I'll use named "Bondo" is the resin, the body filler, glazing putty is crap. I use U=Pol and Evercoat Rage.

Roger Walling

Your car is SMC. SMC resin can be used on older hand laid up cars.

Older hand laid up cars are more brittle than SMC cars.

20 to 25 minutes seems a little too long for me. I usually expect it to start harding in about the time that I get the last piece of glass on. This prevents the glass from sliding off the car.

(2" wide masking tape will stop it from sliding if it is getting hard to slowly)
Experience and ambient temperature will guide you.

If it gets bubbles and smokes when it gets hard, you put in too much hardener.

One way to extend the working time is to have the resin in a flat pan so that the heat of reaction is not contained in a close space.

I always use a sq. plastic antifreeze container with the side cut out.
You can easily dip the glass in it and slide it out on the edge to remove excess resin before putting on a sheet of cardboard to work in the resin with a brush before applying it to the car.

Too much resin is not good as the strength is in the glass, not the ressin. If you don't have quite enough resin on a layer of glass, you can use more on the next layer and work it in.)

PS, I never use fiberglass cloth unless I really want to build up a very strong area and I always cover it with mat. . Use random wove mat instead on most repairs.

If you try to feather out cloth on a finish panel, it will always show and give you trouble with "wicking" when you apply primmer and paint.

I could go on forever doing fiberglassing but my fingers are tired from typing. Ask more questions if I did not make myself clear.

csherman

Your car is press molded
I like the 3m brand of resin
little more expensive
watch how much hardener you use.
If it gets bubbles and smokes when it gets hard, you put in too much hardener.
Depending on the temp and what I an doing and how much I mix
I use 10 drops per oz......
Less if I need it to cure slow or doing a large area or if it is hot outside - or it will kick fast

DansYellow66

I don't know the exact year but SMC didn't appear on Corvette body panels until around 1980. I think there are a few SMC repro body panels in the market for C2, not sure.

Roger Walling

I think that it was 55 or 56? Don't quote me.

csherman

SMC wasnt used until the late 70's / 80's

http://www.autocomposites.org/composites101/history.cfm

csherman

1972 - 1972 Chevrolet Corvette body panels are converted to SMC using Owens Corning Materials.

wombvette

The first SMC panel used on a Corvette was rear fenders in 71. There was an earlier material called Gennite used starting with later 69 doors. SMC panels were then worked into the body up to 74 when the last remaining panel was applied which was the rear deck.

65ZR1

Just to clarify. The 3m/Bondo brand was working great. It cured nicely and was easy to work with. However it is a laminating resin and thus cures with a tacky surface. Im doing a modified car with lots of Fiberglass mods so the extra sanding time was adding up to quite a few hours.
The waxed resin I purchased is very easy to apply and is curing nicely. The waxed surface is very easy to sand and is exactly what I wanted. The problem is that it cures with voids that I'm assuming are air bubbles. They don't appear to be there when I'm laminating but are there the next day. The new resin is very thin and runny, I'm not sure if that is a contributing factor.
Thanks for all the replies so far! But I'm back to using the old resin until I can figure this out.

DansYellow66

The only thing I can add is that you might experiment with trying to increase or decrease the resin applied to the mat. Bubbles sounds like the matrix may be ending up a bit too resin rich and bubbles are forming in the excess on top. If a solid layer of resin is forming over the surface after rolling it out and you are seeing the bubbles in that - that would sound to me like too much resin is being used. But, could be that it's actually resin lean if the bubbles could be areas that didn't bind together when rolled out. If you could take a picture of the bubbles that might help someone figure it out.

Roger Walling

If the first Corvettes are hand laid up, and the latest ons are SMC, what are the 1957 materials called?

csherman

53's are hand laid cloth
54's on up are press molded
57 are also press molded.

csherman

65 ZR1 - are you using a roller or anything to press out air bubbles?

wombvette

I think you are confusing processes with materials. Lots of materials have been used in Corvette production. They include epoxy resins, polyester resins, and heat set materials, one of which is SMC. There have been several processes used including hand laid, vacuum bag, matched mold, and injection mold. All when combined are fiberglass reinforced plastics (FRP). SMC is just one of those materials that by definition is a sheet of material with fiber included that is rolled out and laid into the matched mold.

Bill32

What's the pat number on the 3M can? I'm asking this because the 3M I'm using cures rock hard on the surface.

What's the ambient temperature in your shop?

You mentioned using a roller, like this?

Roger Walling

I just had to do some research on this and this is what I found

"In this report, we compare and contrast the properties of the two major
material forms used in compression molding of modern Fiber
Reinforced Plastic (FRP) products. Compression molding of FRP in
matched metal dies is the most cost effective method of producing high
value composites in production volumes from thousands to millions of
pieces annually. This process has the best combination of cost, quality
and properties of all composite forming methods. The two major
material forms are Liquid Composite Molding (LCM) and Sheet Molding
Compound (SMC). Both forms take advantage of the benefits of FRP
including; corrosion resistance, high strength to weight ratio, dimensional
stability, parts consolidation, dielectric strength, minimal finishing, high
repeatability, low tooling costs, and design flexibility.
The principal difference between LCM and SMC compression molding is
whether or not the fiber reinforcement moves, or flows, as the mold
closes to form the part. LCM uses a variety of preformed fiber
reinforcements that are positively placed in the mold exactly where they
need to be in the final molded part. A measured charge of liquid
resin paste is placed on a portion of the reinforcement and a controlled
closure of the press flows the paste throughout the stationary
reinforcement to fill the mold. In contrast, SMC is a leather-like,
pre-manufactured sheet combining fiber reinforcement and paste that
is cut and stacked to cover a portion of the mold. The controlled closure
of the press flows the compound (fiber reinforcement as well as paste) to
fill the mold cavity. In other words, the LCM process flows the resin
through stationary reinforcing fibers to fill the mold cavity and the SMC
process flows both resin and reinforcing fibers to fill the mold cavity."


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