Motorsport Composite Fundamentals: Surface Finishing

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Surface Finishing

04.01

00:00	The surface finish of our pattern will be transferred to our mould surface and then to our part surface.
00:06	And while it can be improved at each stage, the better we can make it early on, the less work we'll have to do.
00:11	A smoother surface also generally leads to an easier release.
00:15	Another reason to aim for a good surface as early in the process as possible.
00:20	We'll continue using our CRX fenders as an example.
00:24	As with most body panels, we're aiming for the best possible finish.
00:27	In the previous module, we covered spraying primer onto these to seal the porous surfaces of the body filler.
00:33	But also provide some material to cut flat.
00:36	We've specifically used the high gloss surface primer variant of the Duratect product rather than the easy sanding alternative.
00:43	Because it's slightly harder and allows us to polish to a higher gloss surface finish.
00:48	So, let's dive straight in, starting at 400 grit with a sanding block.
00:53	With a dark colour primer like this, it's fairly easy to see the areas that are flat and smooth and those with high and low spots that still need more sanding.
01:01	In some cases, with a lighter colour primer, it can be harder to identify these areas.
01:06	For this reason, bodywork and paint shops would use a guide coat.
01:10	Put simply, this is a thin coat of paint in an opposing colour to the primer that is dusted on.
01:15	As we sand the panel with a long sanding block, the guide coat will be sanded off the high spots and remain on the low spots.
01:22	If we can't sand them out, we'll likely need to turn back to more panel beading and body filler and then of course another coat of primer.
01:30	Remember, with any of this material removal, we want to retain a layer of the primer so it can do its job and seal the porous surfaces beneath.
01:38	Burning through slightly in some small patches on the edges is inevitable, but we don't want to leave large uncovered areas of bare filler.
01:46	Once fully sanded with our 400 grit, we'll gradually work up to 1200 grit wet sanding.
01:51	This is a relatively good surface finish for a pattern already, but there's no point leaving it at good enough, so we'll follow up with a cut and polish.
01:59	The scratches from 1200 grit are about the threshold of what most cutting compounds will be able to remove in a reasonable amount of time.
02:06	If we have 2000 grit sandpaper, then working up to this is going to be ideal.
02:11	Cutting and polishing is a deep rabbit hole that some of you as car enthusiasts might have already gone down.
02:16	While there are a range of different products available for different surfaces, often depending on their surface hardness, we're going to keep our discussions very basic and use whatever automotive cutting compounds and polishes we already have on hand.
02:29	In this case, we're using a foam pad on a polishing machine, but as always, if you're limited to an application pad and some elbow grease, this can still achieve great results.
02:39	It's just going to require some extra effort.
02:41	After the cutting compound, we can move to a less abrasive polishing compound.
02:46	Depending on the thickness of the primer available, we could keep cutting and polishing the surface to achieve a higher gloss finish.
02:52	But at some point, we need to know when to call it done, as we still need to do some work on the mould surface and on the final parts regardless.
03:00	At this point, we have our fenders fully primed and polished to a reasonably glossy surface finish, so let's wrap up with a summary before moving on.
03:08	Our surface finish of our pattern is transferred to our mould and then to our part, so we want it to be as good as possible.
03:15	Typically, our patterns will have a coating, like a primer, that we can sand, cut and polish to a high gloss finish that will not only look better, but help with release.
03:24	This is a relatively simple process, starting at around 400 grit and working up to at least 1200 grit wet sanding.
03:31	Using a guide coat can help identify high and low spots as we sand and will generally work with a long sanding block on the open surfaces of body panels.
03:40	After sanding, we'll follow up with a cutting compound and then a polishing compound, ideally using a polishing machine.
03:47	While there are many products on the market available for different surfaces, to keep things simple and accessible, we recommend a general automotive product that you also like to use on your car.