Motorsport Composite Fundamentals: Repairing Fibreglass Parts

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Repairing Fibreglass Parts

07.33

00:00	In the previous section of the course we looked at the construction of composite parts using wet lay techniques, also known as hand lamination.
00:07	In this section we'll be looking at the repair of composite parts, which will also be using wet layups.
00:13	The process is similar between fiberglass and carbon fiber parts, but has a few significant differences.
00:19	So, we'll look at fiberglass repairs in this module, and carbon fiber repairs in the next.
00:24	Showing an example of a real automotive part in each.
00:28	Before we get into the practical work, this is an extremely dusty process involving cutting composite materials.
00:34	So, it's recommended that we work in a suitable environment, be it outside or a part of the workshop that we don't mind getting covered in dust.
00:42	It's also critical that we have the correct PPE, gloves, eye protection and a respirator should be worn throughout the entire process, with hearing protection added for when using power tools.
00:52	Coveralls are also a good choice if we want to save our clothes, and also cover any exposed skin as the dust is extremely irritating.
01:01	As an example, we'll be looking at an aftermarket fiberglass bumper from a Mazda RX-7 race car.
01:07	This part has had an impact and cracked in a few areas, the worst being around the brake cooling duct.
01:12	The wrap has already been removed from this part, and we just need to repair the fiberglass so it's strong and smooth enough to be wrapped again.
01:19	If the part is dirty from life on a race car, we first need to give it a good clean using a product like Simple Green or soapy water, and ideally a pressure washer and a good scrub with a scotch pad.
01:31	A key idea behind most composite repairs is similar to that of rust repairs in steel or rot in wood.
01:37	We want to remove all the rust, rot or in our case broken and delaminated fiberglass before patching it back up.
01:44	If we leave an area of delaminated fiberglass and patch over it, it'll still be a weak spot so it'll likely spread like rust through steel.
01:53	These areas are visible in fiberglass as the damaged area turns white and more opaque, where there is fractures and delamination.
02:01	In contrast, a solid laminate generally appears more of a dark and deep tone.
02:05	We start the repair on the backside of the part, repairing this first to provide a solid foundation that won't be visible.
02:12	To remove the damaged material we use a fiber disc on a die grinder or similar rotary tool and grind all the damaged material out of the backside only at this stage.
02:22	We can then tape over the front side surface over the damaged area.
02:26	In this case we use race tape but anything reasonably strong and not overly adhesive will work.
02:31	This tape prevents any resin from dripping through into the front side, keeping it tidy.
02:37	If it did drip through though it could create a pocket of cured resin with no reinforcement on the front side.
02:43	And then when grinding this flat it could make a hole.
02:46	After grinding we'll change gloves before we start laminating to avoid contaminating our layup with the grinding dust.
02:52	This dust won't really cause any issues but it's nicer to work with a relatively clean surface, so use compressed air or a rag to give it a quick clean without wiping any more contaminants into our repair area.
03:05	Next, we'll prepare for lamination.
03:07	Fiberglass repairs are typically done with 225 or 450 GSM chop strand mat, the edges of which can be torn or cut.
03:15	Cutting naturally leaves a defined edge that looks tidy, but creates a hard edge on the part where the strength changes.
03:23	Theoretically this could concentrate stress, whereas tearing an edge creates a more consistent transition but also looks messier.
03:30	I say theoretically because in practice it doesn't really make much of a difference for a basic CSM part.
03:37	So, we should just use whatever method suits what we're working on.
03:41	Next, we use wax polyester resin for the repair since we'll be working in a single stage.
03:46	While we could use vinyl ester it's much more likely that this part was originally made with polyester resin and it's also cheaper and easier to use.
03:53	We can't use epoxy for repairs with emulsion bound CSM because it doesn't dissolve the binder.
03:59	Remember epoxy can only be used with CSM that's powder bound.
04:03	The resin is catalyzed as per usual at 2% with MEKP and mixed thoroughly.
04:09	The process from here is just like our lamination for the construction of composite parts.
04:13	We apply the resin to the surface, lay up the CSM and wet it out.
04:17	Alternatively it is possible to wet out the reinforcement on a board first and then transfer it onto the surface.
04:23	We want to build up at least 3 layers so we have a good amount of material to be able to grind into from the front side which we're going to do in the next step.
04:32	Once in place, fully wet through and consolidated with the brush or a roller, we need to leave the backside repair for about 3 hours to cure.
04:41	This isn't a full cure but it's past peak exotherm and long enough to start working on the front side.
04:47	Now, we remove the tape and grind the material on the front side with the same fiber disc down until we don't see the white damaged laminate anymore and we're getting into the solid laminate that we just created.
04:58	Again, we need to remove all the damage but we don't want to burn through our new material either.
05:04	A resin bog thickened with cabosil can be used to fill any deeper irregularities in the surface and ensure we don't get any voids in our repair.
05:12	A bit of bog is much better than voids and there's not really any downside if we're talking about a repaired CSM part that's now far from optimal.
05:21	Then it's rinse and repeat, wet laying and consolidating enough layers of reinforcement to come proud of the original surface.
05:28	We don't want to overshoot it by too much as we'll just make more work for ourselves in the next steps.
05:33	At this stage we leave the part overnight to allow the repair to fully cure.
05:38	Then we can grind the front side down to match the original contour of the bumper.
05:42	The bulk of the material can be removed with the same fiber disc on the die grinder.
05:47	But as we approach the original shape we'll switch to a dual action sanding machine, also known as a DA machine, for more control.
05:54	Anything between 80 to 400 grit should be suitable.
05:57	At this stage our repair work is basically done and we're entering the realm of auto bodywork.
06:03	Which of course isn't the focus of this course, so we'll gloss over some of the finer details.
06:09	To finish the part we use a lightweight body filler, mixing thoroughly with the hardener as per the supplier's instructions.
06:16	And then applying it over the front side surface to fill any remaining imperfections.
06:20	After this is dried we can sand back the excess again to match the original bumper shape.
06:26	Continuously checking with our hands to feel the surface profile and not remove too much material.
06:31	We then also sand a larger area around the repair with 400 grit and spray on a primer to seal up the repair.
06:38	Some more sanding and smoothing later to match the surface finish and the part can now be painted or wrapped to suit the car.
06:45	So, let's summarise the key things to remember from our repair.
06:48	Before we start any composite repair we need to ensure the part is clean, we're working in a suitable area and have the correct PPE.
06:56	For our fibreglass bumper we started by grinding out all the damaged laminate from the backside, cleaning the area, taping the front side of the part and then wet laying a solid foundation on the back with CSM and polyester resin.
07:10	Once this was cured we shifted to the front side of the part, removing the tape, grinding away the damage and then laminating proud of the original surface of the part.
07:18	We then used a grinder and DA sander to match the original surface with some body filler to fill irregularities and primer so the part is now ready for whatever finishing processes will follow.