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MNBoxster · 02-21-2007, 04:10 PM

Hi,

The OEM Bumper covers are made of Urethane a pliable plastic which can absorb the energy of the impact and then return to it's normal shape. The fibreglass parts are attractive because they can be made easily in a mold, and can be relatively thin, making the piece cheaper to produce and cheaper to buy. But, bye-and-large, they are less durable.

Fibreglass is made up of resins with short glass firbres imbedded into it. It is fairly strong, but does not mold to a smooth, paintable surface. If the component was Fibreglass alone, with the weave (Glass fibres) just below the surface, painting it would leave a very irregular, rough surface. So, most fibreglass components are usually coated with a polyester resin called Gel Coat. Gell Coat is usually sprayed into the mold first (after a mold release agent is applied directly to the mold such as a wax or silicone, so the part will separate from the mold once cured) and gives the component a smooth, paintable surface. This Gel Coat, though smooth and relatively durable, is prone to dings and scratches. Unfortunately, what you describe is characteristic of the Gel Coat pitting, in addition to losing the paint.

Gel Coat is somewhat difficult to work with and the solvents used in it can be toxic. In order to repair this piece, you'd first need to fill the pits, sand it smooth and then apply the paint. Bondo or other filler can be used, but usually makes a poor bond, so with time, it's likely to come loose. An old trick used by guys with fibreglass cars such as TVRs and Lotus, use a plain two-part epoxy to fill the pits. Epoxy has several advantages. First, it cures fairly quickly, whereas Gel Coat and Bondo continue to off-gas their solvents for days, causing a delay in painting the piece. Epoxy also has very little (if any) shrinkage, so multiple applications are usually unecessary. It can be roughed in and stays flexible for some time before curing. Then, it can be sanded smooth. It bonds well with the piece and accepts paint well too.

There are a number of rubberized coatings on the market, which are paintable. These are often used on rocker panels and areas prone to stone chips. The problem is that these tend to be rough and aren't easily smoothed out to paint quality.

So, unless you don't mind an irregular finish, the fibreglass piece will require some fairly regular maintenance. The Air Dam on my Lotus Esprit experiences much of what you are. But, it is very thick, and so can withstand more abuse. I use the epoxy method to repair it and and then paint it w/ touch-up paint. This looks pretty good. But, every 3-4 yrs., the accumulation of these chips makes it necessary to strip and sand the piece and do an entire repaint. This is the downside of the cheaper fibreglass pieces out there...

Happy Motoring!... Jim'99

02-21-2007, 04:10 PM	#2
MNBoxster Registered User Join Date: Sep 2005 Location: Minneapolis/St. Paul, Minnesota, USA Posts: 3,308	Hi, The OEM Bumper covers are made of Urethane a pliable plastic which can absorb the energy of the impact and then return to it's normal shape. The fibreglass parts are attractive because they can be made easily in a mold, and can be relatively thin, making the piece cheaper to produce and cheaper to buy. But, bye-and-large, they are less durable. Fibreglass is made up of resins with short glass firbres imbedded into it. It is fairly strong, but does not mold to a smooth, paintable surface. If the component was Fibreglass alone, with the weave (Glass fibres) just below the surface, painting it would leave a very irregular, rough surface. So, most fibreglass components are usually coated with a polyester resin called Gel Coat. Gell Coat is usually sprayed into the mold first (after a mold release agent is applied directly to the mold such as a wax or silicone, so the part will separate from the mold once cured) and gives the component a smooth, paintable surface. This Gel Coat, though smooth and relatively durable, is prone to dings and scratches. Unfortunately, what you describe is characteristic of the Gel Coat pitting, in addition to losing the paint. Gel Coat is somewhat difficult to work with and the solvents used in it can be toxic. In order to repair this piece, you'd first need to fill the pits, sand it smooth and then apply the paint. Bondo or other filler can be used, but usually makes a poor bond, so with time, it's likely to come loose. An old trick used by guys with fibreglass cars such as TVRs and Lotus, use a plain two-part epoxy to fill the pits. Epoxy has several advantages. First, it cures fairly quickly, whereas Gel Coat and Bondo continue to off-gas their solvents for days, causing a delay in painting the piece. Epoxy also has very little (if any) shrinkage, so multiple applications are usually unecessary. It can be roughed in and stays flexible for some time before curing. Then, it can be sanded smooth. It bonds well with the piece and accepts paint well too. There are a number of rubberized coatings on the market, which are paintable. These are often used on rocker panels and areas prone to stone chips. The problem is that these tend to be rough and aren't easily smoothed out to paint quality. So, unless you don't mind an irregular finish, the fibreglass piece will require some fairly regular maintenance. The Air Dam on my Lotus Esprit experiences much of what you are. But, it is very thick, and so can withstand more abuse. I use the epoxy method to repair it and and then paint it w/ touch-up paint. This looks pretty good. But, every 3-4 yrs., the accumulation of these chips makes it necessary to strip and sand the piece and do an entire repaint. This is the downside of the cheaper fibreglass pieces out there... Happy Motoring!... Jim'99 Last edited by MNBoxster; 02-21-2007 at 07:43 PM.