Adding lightness through sanding, Questions about body repair and paint

[BDA]

I am no expert on fiberglass or fiberglassing so I could be wrong or I this could be ridiculously obvious but because I'm not an expert, I can't judge that but I've always thought that a primary reason for veil is to be a resin carrier. It obviously provides some strength but holding resin seems like a main purpose and in a way makes a sort of gelcoat surface. Resin is not only among the fibers of the veil but above it and I think that layer of resin is what you actually want to end up sanding to provide a good smooth surface for your paint.

Just my thought about it...

[GavinT]

Veil is intended (and used) in a mould situation to mask (diffuse) the coarse CSM underneath. The heavier weights of CSM are considerably courser so it probably makes sense in a production environment to lay down one layer of veil plus three layers of heavy CSM instead of eight layers of lighter CSM. That'd be my speculation anyway, and that veil provides some 'insurance' against print through. Also, the moulding process usually includes gelcoat as a finished surface which can be relatively thick, too.

I guess someone came up with the idea of using veil post-production and applied externally on our cars with not so much the masking objective, but of encapsulating cracks.
Needless to say, these are entirely different use cases with entirely different objectives.

So, yeah, this application of veil post-production inevitably makes it a 'resin carrier'. That's unavoidable given the viscous nature of the resin and the lightness of the veil. We end up with a resin rich layer on top. Or, to put it another way, the additional surface layer essentially creates a thicker panel but with less reinforcement.

To me, that's acceptable provided the cracks are arrested.
In practice, the next step is to prepare that newly created surface for the paint stages. To do that, we need to scuff the surface at a minimum in order to provide a key and to eliminate the wax on the surface. The question then becomes; how can that scuffing be accomplished without breaking through that poofteenth (technical term) of veil.
There's other issues like how to deal with the seam created when overlapping sections of veil. While it might be possible to cover the bonnet, doors or engine cover in perhaps two sections (probably one for the doors) that's not the case with the majority of the body.

What's actually happening here becomes a conflict of objectives; we need to perform the final shaping/scuffing of the body and yet, not break through the veil.
It's with the above in mind that I think the best practical step is to apply spray polyester filler as the base for that final shaping instead of attempting to shape at the veil stage.

But that's just my take.

[BDA]

You pretty much explained what I did: I covered the engine cover and bonnet and IIRC both doors. I didn’t have any stress cracks but I really don’t remember why I thought it was necessary. There could have been some printing from thin cloth or there was some general roughness. I used fine sandpaper on a DA orbital sander so as not to get into the fibers of the veil. I believe my final grit was 600. It seemed to work pretty well. The paint shop kept my car for what seemed like a long time. They didn’t mention any extra work they needed to do but they may have figured that into the estimate… or they may just have been jammed up.

I didn’t try to cover the cloth printing on other parts of the car so I didn’t need to worry about seams but I think if I had, I would have butted the pieces of veil and filled in the line with something - resin, micro balls, or other body filler.

So that was my amateurish method. Thanks for filling in the technical details didn’t know.

[Kendo]

My first experiment with veil was covering the front trunk lid. A PO had repaired the front edge with cloth and I wanted to cover it. I used two sheets overlapping on the front/back axis, and had a hell of a time sanding down the overlap. So in future lapping applications, I cut veil to shape and picked the edges, pinching off small bits along the edge. That basically feathers the edge. So then any overlaps are practically invisible. It works for single pieces too, feathering the edges into the old surface. Using one of those rollers that look like a stack of washers also helps to push the fiberglass down to the bottom of the resin.

[Dilkris]

Digressing slightly but on the same subject - what is the difference, (in simple terms please), between polyester, epoxy and we now have it seems "isophthalic" resins (the later championed by Banks), and which should we be really using for localized repair works?   

[gideon]

If you choose polyester resin then I believe that isophthalic is a better choice than orthophthalic.  It has better mechanical properties and some other advantages, but the main practical advantage for repair work is probably that isophthalic resin shrinks less than orthophthalic.  I thought this was a fair summary

https://www.ecfibreglasssupplies.co.uk/what-type-of-resin-shall-i-use

[GavinT]

Well, this link says the opposite!

https://aronpolymer.com/isophthalic-vs-orthophthalic-polyester-resin-which/

But whatever the truth is, I don't think it matters much. The shrinkage issue is of interest to manufacturers of FRP moulded products that look to hold tighter tolerances.
In our application we're more concerned with patch repairs which will be sanded down anyway, so, not a problem IMHO.

[RonPNW]

I'm new to this but my reading indicates that the shrinkage can result in cracking at some later date. It is usually due to some temperature extreme or sitting in the sun. That is one of the reasons that all my repairs and filling is done with epoxy.

I have also read that a few coats of epoxy primer may mitigate shrinkage in layers below the primer.


To be fair, I have no direct knowledge but the info did come from one of the pro level bodywork forums.

Ron

[EuropaTC]

Well, this link says the opposite!

https://aronpolymer.com/isophthalic-vs-orthophthalic-polyester-resin-which/

But whatever the truth is, I don't think it matters much. The shrinkage issue is of interest to manufacturers of FRP moulded products that look to hold tighter tolerances.
In our application we're more concerned with patch repairs which will be sanded down anyway, so, not a problem IMHO.

Gee, I'm confused now !

My knowledge in this area is at least 20yrs out of date.  In my working life we did specify & use GRP vessels and I was fortunate to work with a couple of fabricators who also had technical bods on their payroll. So I did learn a bit about the resins they/we used and more importantly how easy/hard it was to fabricate with them.  As this was the chemical industry we were more concerned with chemical & mechanical properties so we tended to use vinyl esters with a cheaper polyester resin for the external (non chemically exposed) layers to keep fab costs down.

I'll be honest and say that up to this juncture I'd have said Isophthalics were the ones for us but as to what Lotus used in the 70s, well I think that would be whatever was cheapest at the time. A car body doesn't have a lot of stress or chemical attack, it's just there to keep the rain off the driver.

Anyway I read both links and the second one appears to contradict itself unless of course I haven't had enough caffeine yet to be thinking clearly. The screenshot shows a description and from my limited knowledge, shorter  chains usually means better chemical stability, stronger, heat resistance, etc. It says OPs with longer chains have lower melting points and by inference, lower heat resistance which I understand.  So IPs are "better" ?

Then a paragraph later it goes on to say OPs have higher heat resistance ?

Here's where the luddite comes in and I'm not convinced that in repairing a 50yr old Lotus, it makes any difference what you use. They all shrink a bit and from what I remember the catalyst/accelerator choice and ratios can make or break your repair.  Miles Wilkins often said that repairs should be left for much longer than we'd like to settle (shrink) before sanding down or painting. His turnaround was several weeks for repairs and I think the experts in this field do the same today.  So if you're leaving it for 3-4 weeks then does it really matter which one you pick ?

Brian

[GavinT]

Agreed, Brian. Mea culpa . . . my link is both confusing and contradictory. I should have examined it more closely.

Ron, my presumption is that Colin used the cheapest Polyester he could find. I'm not convinced cracking can be attributed solely to the resin used and cursory examination of a bare shell will reveal what we already know - the hand laminating was done by relatively unskilled people. Some of the factory's efforts to fill defects were truely dismal.

That's not to say they didn't do an OK job, but it is what it is, as they say. There's thin sections, thick sections and pools of resin which subsequently and inevitably crack - some did on mine. To be kind, this could be considered a testament to the forgiving nature of the material, but these bodies are also 50 years old.

I favour the Polyester system because it's the well trodden path with a depth & breadth of knowledge, experience and related products not always easily found with the Epoxy system - and better the devil you know, too.
There also seems to be ample info on painting boats with Epoxy substrates but precious little re cars.

But I reckon one could largely adapt an equivalent Epoxy system.
I'd want to find some CSM with powder binding in several different weights. I believe there are non-woven veils that are Epoxy compatible.
Epoxy fairing compounds are readily available but I haven't seen anything equivalent to the 'glaze' type Polyester based automotive products.

What's the procedure from there?
Does the new Epoxy surface require a primer? Presumably it'd be epoxy primer (presumably non-etch?) over the whole car and then Polyester spray filler for the final shaping followed by urethane 2K primer and the top coats? - dunno and there's others with better knowledge than myself on this aspect and this enquiring mind would like to hear that.

So, I tend to think the shrinkage angle draws rather more attention than is deserved and doesn't come into play in any practical sense as Brian lays out.

[dakazman]

   I'm staying out of this because I only can comment on polyester resin repairs.
 I am learning a lot though, as EuropaTC says, my experiences are 20 years old.
  Shrinkage after painting, on panels that haven't been repaired are showing flaws.
 Flaws in fibers rising in OEM sections, In my opinion these cars have been through a lot of environmental changes, in that they all have hidden internal flaws. Covering an entire panel with either epoxy or polyester resin will probably mask the underlying flaws, encapsulate them, so to speak for the next generation. We can only preserve history so much. WTS, lets document these repairs and compare notes in a year or so.
Dakazman 
 

[Dilkris]

This whole thread reminds me of the time I asked somebody the time - they then proceeded to tell me how to build a clock.
Whilst this was interesting, at the end of an hour, I still didn't know the time.