Fiberglass Repair

  Home Up Fiberglass Repair Polishing Paint Mold Making Composites Bonding Wiring Hand Forming Riveting Drilling Cleaning

Note:  A listing of publications follow this

Fiberglass Repair

Sure there's a lot of information out there concerning fiberglass....just not the stuff you really need to know.  We'll try to lay down some general rules to go by as well as give you some good information on ideas you may want to consider for other adhesives, epoxies and glues.  We'll make a point of categorizing things so that you can come back for quick reference...if needed.

Fiberglass work isn't nearly as difficult as many think.  Its also very forgiving.  You can mold it, sand it, cut it and pretty much re-do anything as many times as you want.  If you make a mistake, you always have the option of cutting it out and starting over again.

Standard Automotive Fiberglass types:

Basically, there are two types of glass cloth you'll see at your local body repair shops or automotive sections of Walmart, Kmart and so forth.  These are "Matt Mold and standard woven cloth.  Both come in varying grades (thickneses).  Matt mold is used for body work because of its random pattern.  It won't leave a noticeable 'weave' in your paint finish.  Woven cloth is primarily used for building up structure or splicing sections of products made of the same material.  Although woven cloth has better strength and crack resistance, Matt mold gives body workers an advantage of being able to work areas without the fear of sanding down to the visible weave of the woven cloth.  Ideally, a repair would be made by using woven cloth and then using matt mold to cover the repair area.  If you consider that bodywork always requires that the repair be blended in to the rest of the surrounding areas, you'll realize that a deep depression would have to be sanded wherever the woven cloth was to be used. Because of this, automotive body workers use matt mold for nearly all repair work.   They get the strength they need by assuring that they use an adequate thickness.   So, unless you're planning on building a wing spar for an aircraft, use matt mold.

Matt mold is notorious for getting in your skin.  If you don't cover your hands and protect your lungs, you'll have all those little glass splinters haunting you for a long time.  Where a mask, gloves and coveralls when using it.  Also, use an old household vacuum to run directly behing your sander when you do the sanding.   This cuts down on 90% of the dust and slivers which may otherwise find themselves all over your working space.  Try the vacuum works!


Resins are simply the stuff that you put on the glass cloth to make it hard.  

Resins come in different types.  GM decided to switch to epoxy in the early 1970's.  Although polyester may work for repairing later models, you may consider trying to locate a suitable epoxy resin before doing extensive bodywork on 1971 and later models. (check this to make sure) Polyester resin is used extensively in automotive repair for fiberglass and metal vehicles.  Its adhesion and workability are good and it dries fairly quickly. It is sensitive to moisture and contamination of the surface it's being applied to so make sure you have a fresh, clean surface before you apply it.

Some characteristics:


Longer drying time - tough (not brittle if mixed properly) - harder to find -more difficult to sand - not as sensitive to mixing or surface contamination - doesn't soak into glass well unless warm. (In cold weather it sucks) - Flexes more without cracking.


Dries quickly - sands easily - cracks easily if mixed too 'Hot' (Brittle) - sensitive to        moisture and contamination - stinks - drips (more liquid) - soaks better - easier to find.

Most of the resin you'll find will be polyester base.  Its the stuff in the cans which you can find everywhere.  It has a distnictive odor common to boat yards or surfboard shops.


Fiberglass repairs are fairly straight forward. Good adhesion, proper depth and an adequate bearing surface are what's required to assure original strength. In addition to this, attention must be paid to proper mixing of the resin, surface preparation and cleanliness as well as the grade of fiberglass cloth used.

Proper Depth:

Proper depth and bearing surface is accomplished by sanding a "V" or concave pattern in the fiberglass.  The center of this "V" should be the crack itself. From the center, the sanding pattern should taper outwards to the surface.  The taper gives you your bearing surface.  The amount needed to assure adequate strength will be determined by the size of the crack as well as the depth of that particular area.  Obviously, if the crack is large, you'll want to sand the taper out more.  Normally, a good taper would be as far out from the crack as reasonably possible. During the final repair, pieces of glass which increase in size will be used to fill the canyon. (The "V" you've created)

Adhesion will depend on the surface preparation and cleanliness as well as the texture. Keeping things absolutely clean and free of water and oil is absolutely essential to a good repair. The texture of the prepared surface shouldn't be too smooth. For repair purposes, 40 grit paper or even a course file is fine for repairs to the back sides of the fenders. As long as you don't have it so rough that the glass can't get down without trapping air in the repair.  For exterior repairs, don't worry about using anything beyond 120 Grit. You can use 80 but be careful not to stray too far from your repair.  You want to avoid breaking through the Gel Coat in the adjacent areas.

Grade of Glass:

For major repairs the best method for determining this is to go and look at what was there originally.  If your doing small patch work you may want to go down a notch or two on the thickness.  Remember, you're trying to duplicate the strength of the original panel. Inadequate thickness or fiberglass weight may not accomplish this.  Obviously, trying to fill small holes or cracks with stiff, heavy "Matt Mold" wouldn't work very well.

Small Cracks and Holes - Filling:

The best method for repairing minor defects in fiberglass is to prep the surface as explained above.  Holes can be prepared by "countersinking" using a countersink or drill bit.  Chop the fiberglass using a pair of scissors.  To do this, simply fold the cloth, cut off the edges and trim off the edge bit-by-bit.  Do this over a bag or other container.  Now, you'll have a bag full of chopped fiberglass for the future. After properly mixing a cup of resin, drop 'pinches' of the cloth in the cup until you have the right consistency.   Using an 'Acid Brush', scoop out enough for the area you're repairing. (Trimming the acid brush bristles really makes a big difference in managing this). After getting the glass on the repair area, dab the acid brush vertically into the repair. Keep packing the glass down, assuring that it's all in contact with the surface and that air bubbles are forced out. Let it dry and sand as necessary for your finish. 


If you haven't had any experience with mixing resin and doing fiberglass work do this: Go down to your local supermarket and buy some small paper coffee cups.  (Don't ever mix resin in the cups that have a wax barrier. The wax will dissolve into your work.) Measure an equal amount of resin into each cup.  You don't need to fill the cup.   Use an amount which will require a measurable amount of hardener.  Because polyester hardener is powerful and only used in drops, you want to be able to count the drops you put in each cup.  Normally the instructions have you put about 15-20 drops of hardener for each ounce of resin.  Picture a one ounce perfume bottle.  For the resin, eyeball 1/4 - 1/2 of this into the cups.  Now add the hardener in varying amounts beginning with what you think is correct.  Cup 1 should have an accurate amount.  Cup 2 ......a little more.  Cup three....a little less.  Now you want to make comparisons of the cure times and characteristics of the resin following its cure.  Is the resin in cup #1 too brittle?  Did it cure properly?.  What happens when you add too much hardener... or not enough?  Consider spreading some resin on a piece of cloth or somewhere else.  Make notes of the same characteristics.   Bend it, sand it, touch it and see if you feel satisfied that it will serve your repair or bodywork well.


Heat makes any adhesive cure faster.  If excessive heat is used your repair will become brittle.  If resin is applied in a cold work place, it may or may not cure 100%.  For instance, lets say your out in the garage on a cold night.  You decide to mix a normal batch of resin using your standard mixing amounts.  After your finished doing the repair, you leave the garage for the night.  The next morning you come back and find that your repair has a nasty, sticky feel to it.  This is an example of polyesters sensitivity to moisture and temperature.  Although the repair has cured in the middle as a result of the heat developed by the chemical reaction, the surface remains sticky.  The lower temperatures also caused moisture to condense on the surface of the repair.   There are only two things which could have been accomplished to prevent this.  More hardener could have been added...making the majority of the repair too brittle.  Or, the garage could have been heated.   When you do fiberglass work, keep the area as warm as you can afford. 

For quick repairs or anything else not requiring the full benefits of the particular product your using, use varying amounts of heat with hair dryers or other suitable appliances. ALWAYS KEEP A FIRE EXTINGUISHER WITHIN REACH!   If you want the full benefits of the product your using, make sure that the temperature remains suitable as outlined in the instructions. The cure and final characteristics will be determined by your control.  Heat = More Brittle, cracking, less adhesion and quicker cure.  Normal temparature = tough, resilient, longer cure.  By adjusting your heat, you can get anything in between.

(J. Myers)

Here are some excellent books on the subject..

1.) Fiberglass Auto Body Construction... cover

2.) Fiberglass Repair and Construction... cover                         

3.) How to Build a Custom Fiberglass Body cover

4.) How to Build Fiberglass Hot Rods,... cover                      

5.) How to Build a Custom Fiberglass Body... cover

6.) The Fiberglass Manual : A Practical... cover


Fiberglass & Composite Materials : An...


Fiberglass Repair and Construction...


Up Next


The contents of this site are copyright 2006, Experimental RV-7 & Vetteworks, All rights reserved