How to fill in a large gap? Such as the sidemarker recesses.

I'm putting a 74 corvette bumper on my 78 body. I like the look of it better because it doesn't have those ugly bumperettes. I'm going to get rid of the seam and I need to fill in the large gap where the step from the old bumper used to be.

Notice the step in the later bumper.

...........................Step
.............................V







How do you guys think I should fill in that space? I'm thinking about stapling a piece of matte 1/2" below the surface, laying resin over it, then filling it in with shortstrand fiber polyester resin filler.

I also need to delete the sidemarker recesses. I was thinking about filling them in the same way.

Ground down the top of the panel, mount the bumper and fill the gap with some play doh or equivalent. Then put some layers of fiberglass on top of the play doh. When hardened remove the play do. Then you can glas or fill the underside.

Is filling in the space underneath important? Maybe I could use some foam. And if I just lay a piece of glass big enough to cover the gap, how will it stay in place? I can't lay it over the existing glass because it will add height to the surface.

Get a sheet of glass and lay on some matte and roll it with resin using a roller (the metal ones work great), let it cure and tear the fresh sheet off the glass. Then cut it to size for the opening and bond with epoxy. Use plenty of epoxy adhesive (thick paste like) to glue it in. Then carefully chamfer out the edges and lay a couple of thin layers of glass in to get a uniform surface and then work from there with fillers and such.

Do you want to get rid of the entire gap between bumper and body? Use SMC compatible products (Evercoat SMC adhesive or Duramix SMC glue). The fiberglass resin will not adhere to the SMC panels.
These SMC products are compatible with fiberglass cloth.

The bumper is fiberglass ??

SMC stuff sticks to fiberglass but fiberglass (polyester) resin/adhesive doesn't stick to SMC.

Not sure what SMC is, the bumpers are fiberglass. The resin seems to stick fine to the underside of my hood. I had to repair a hole in it.

Here's a better picture of the gap. Look at the top center of the bumper.

SMC=Sheet Molded Composite

That's pretty big gap there in the center- it looks like the bumper needs to be built up to match the surround. I'll be interested in this one- the only glass/composite stuff I ever did was in airplane school- a very long time ago in a land far, far away

I worried about cutting a piece of glass to the right shape and gluing it in, seems to me that it would be prone to cracking.

Does anyone see anything wrong with my idea? Laying resin on the edges, stapling matte just below the surface and laying more resin. Using the short strand filler to build it up to the surface. Seems to me this would have more strength.

enkeivette said:

Using the short strand filler

Click to expand...

If I were you, I only would use fiberglass matt and resin...no short strand kitty hair crap.

What's wrong with the short strand filler? Bad experience? I'm going to need some sort of filler.

Polyester based resin and long strand fiberglass jelly is fine for the fiberglass bumper. Lay mat on the underside and build up from there.

SMC is what the rest of the car is made of, it has a release agent in it that prevents anything but SMC products to stick.

Lay a piece of mat and fiberglass resin on SMC and you'll be able to pull it right off.

enkeivette said:

Is filling in the space underneath important? Maybe I could use some foam. And if I just lay a piece of glass big enough to cover the gap, how will it stay in place? I can't lay it over the existing glass because it will add height to the surface.

Click to expand...

You will have to grind down the panel a bit to lay a certain amount of glass on top. If done properly it won't be noticed. I would suggest you reinforce the piece that you layed on top also from the underside. Then I would somehow fill the bottom with something like bonding paste as an example and maybe reïnforcing it from the outside.

It could be simpler if you don't want you bumper to be removable.

Maybe others have better suggestions.

My bad, are you telling me that my corvette isn't fiberglass? :twitch: I've already repaired a hole in the hood with matte and resin, seems to be holding up fine.

Here's what I decided to do:



For those of you who have seen a 78/ 79 bumper you can appreciate how much I had to cut this bitch up. Took about 15lbs off too.

enkeivette said:

My bad, are you telling me that my corvette isn't fiberglass? :twitch:

Click to expand...

Some time in the 70's GM switched to SMC.

I searched the web and found a ton of info:

What is SMC?
SMC is the abbreviation for Sheet Molding Compound. Today it is the most important glass fiber reinforced composite material. Parts made of the economical thermoset SMC prepregs are well accepted in the market. Complex shapes with slugged walls, little radius, multiple spline and inserts are possible. The unsaturated polyester resin is the matrix material. The cross-linking reaction of the thermosetting polymer during the compression molding makes the shape of the component part possible.


Technical process
The SMC production can be divided into two processes - the SMC prepreg production and the SMC compression molding.

The SMC prepreg production
Glass Rovings are used for the economical thermoset SMC prepregs. They are re-spooled and cut to a length of 10 - 50 mm (see Fig. 1). The fibers fall on a foil coated with a paste of unsaturated polyester resin. Combinations with continuous fibers are possible. After covering with a second foil on the top the packet runs through a roller drawing frame. In this step the fibers are impregnated with a resin matrix, so that every filament is encapsulated by resin. The semi-finished product can be wound up or cut to plates of specific dimensions. The storage time depends on the recipe and varies from a few days up to a couple of weeks. The prepregs are processed with compression molding. Table 1 shows a typical recipe. It can be varied to meet specific requirements such as class-A-surface, hot-wet resistance or high tensile strength.


The SMC compression molding
First of all the covering foil must be removed from the prepreg. Then, the right weight of prepreg has to be put in the correct position in the open mold. The mold is closed and heated up to a temperature of 150°C. The press load is about 10 MPa. Fig. 3 shows the viscosity-time profile of SMC. On the left side you can see the viscosity increasing during the aging process. On the right side is the viscosity during the compression molding. First, the resin viscosity is decreasing due to a higher temperature. The resin flows and fills the cavity. Because of the high temperature, the cross-linking reaction rate rises to a maximum. Therefore the resin viscosity now grows. After the reaction - the time depends on the size of the part - the mold is cooled down to room temperature. Now, the part is removed from the mold. The compression molding can restart.


Advantages of SMC technologies
The use of glass fibers as a reinforcement leads to the production of strong and cost- efficient parts. Their relatively low specific weight (density 1,6 - 2,1 kg/m³) offer weight saving effects.
SMC parts are non corrosive and have a long lifetime. Due to good formability during the compression molding process slugged walls, little radius, multiple spline, bosses or inserts can be considered.
Design flexibility, excellent dimensional stability and short lead time in production and the possibility of recycling are some reasons for the SMC product acceptance.


This is from a "Google book" and it wouldn't allow to copy and paste the text...:

Please keep an update w/pics. Just out of curiosity, why didn't you cut the 'fangs' off a '75+ bumper and 'hole out' the license portion?

My car's bonding joints (and various holes) were filled in w/polyester resin'd glass. It had stress cracks everywhere. When the paint was stripped off, the resin/mat came off the SMC like slices of bacon. The body prep was done correctly to hold the 'glass (scuffed up well with low grit abrasive). Poly on SMC doesn't work. I'd hate to see you go through the same thing...

The bottom of the 75-79 bumper cover doesn't meet in a smooth fashion like the 73-74 bumper. It would have taken much more modification to make it fit and look nice. Also the bumperettes extend onto the upper lip.

An easy way to do this is to continue the way you started.

If you want to get rid of the seam, then make bonding strips as TT has suggested,
get rid of the flange on the front of the car,
glue the bonding strips to the car,
get rid of the flanges on the new bumper,
grind to fit the new bumper to car,
glue new bumper to car,
vee and feather top side of seams,
fill with bonding adhesive and or epoxy with glass,
prep for paint.

Just like your original fenders are done from the factory.

Now you have me thinking about this! I like the look of that bumper. Are you going to keep the 'battering ram' for support?

010752 said:

An easy way to do this is to continue the way you started.

If you want to get rid of the seam, then make bonding strips as TT has suggested,
get rid of the flange on the front of the car,
glue the bonding strips to the car,
get rid of the flanges on the new bumper,
grind to fit the new bumper to car,
glue new bumper to car,
vee and feather top side of seams,
fill with bonding adhesive and or epoxy with glass,
prep for paint.

Just like your original fenders are done from the factory.

Click to expand...

I don't see a need to grind off the factory mounting flanges. I plan spread glue over the bumper flanges and the body flanges, bolt it together. Groove the seam with a dremel tool and fill.

gkz, the vacuum tank/ frame ext. is long gone. I even cut away half of the bumper hardware to fit the bumper. Check my other recent thread for pics.