Adding lightness through sanding, Questions about body repair and paint

[RonPNW]

My experience in fiberglass comes from years of repairing white water kayaks. Lots of strength and flexibility, beauty, not so much.
The Europa is unique in that all the panels are thin and flexible. Very different compared to a boat or corvette that layup thick rigid panels. I think the repair process should be different because of this.
I’m still working on major cracks (some parking lot abuse and some Lotus design flaws) and trying to get bonnets and doors to line up.
Years ago I did some repair using polyester that developed cracks around the repair. I’m sanding those out and replacing with epoxy. Current epoxy products claim to stick better to polyester, stay flexible and are stronger. I’m finding interior / unpainted panels are fuzzy with exposed fiberglass fibers. Many of the visible paint cracks are only in the paint. That is when you sand to the fiberglass body (and often only to the primer layer) the crack is gone. So here I my process using epoxy resin (Tap Plastics) and woven cloth for the interior and mat for the exterior (the weave of the woven cloth will show through the paint but it is stronger and easier to squeegee out excess resin). The attached picture shows cracks in paint that disappear at the primer layer.
I’m trying to repair any actual cracks from both the front and back side. The usual drill of sanding to a clean solid surface and a few layers of cloth in a tapered stack. Squeegee the repair to remove excess resin to prevent making the repair too stiff.
Reinforce the weak areas in the original build including the rood, roof pillars, wheel arches, rear bonnet hinge area, rear bonnet rear panel and a few others that will show up when sanding off the paint.
I mixed up an epoxy filler (less than 1/16 thick, for the bad areas) using micro-ballons and epoxy resin. It sticks very well, is light weight, stays flexible and becomes inert once cured, but is a serious pain to sand. A few areas needed a layer of thin matt (veil) to bring panels into alignment.

I’m nearly done with major repairs and rework and need helps on the next steps.
Questions:
What are people using as a final thin filler before primer? It seems that most fillers dry hard and brittle. A likely cause of new cracks. I would like to stay with epoxy which stays flexible (at least the one I’m using does).

I’m thinking a high build epoxy primer. The only one that is easily found is Eastwood. Are Eastwood products worth it? I’m sure it’s someone else, rebranded but I don’t know who.

What has worked for primer, paint coat, clear coat (should I have a clear coat?). It seems most paints assume a stiff panel and are not flexible. Are there a more flexible products available?

Thanx
Ron

[Clifton]

I started repairing with epoxy until I found that polyester cannot be used over epoxy as it will not cure. I did the same as you though, mixing my own fillers. Milled fiber, microballons, and cabosil. I used straight milled fiber and polyester to fix door gaps and ground out spider cracks.

I used a black epoxy primer over everything then sprayed a 2k sandable gray primer over that.

I did single stage paint on this and will never do that again. Base/clear is better. Easier to add thickness with clear and you can remove runs without discoloration.

[BDA]

A guy trued up all the sloppy fitting parts of a Europa body. Maybe this will be helpful to you (https://www.lotuscorps.org/wp/truing-up-the-body-fit-on-a-1973-lotus-europa-part-1/).

[Kendo]

My painter used this stuff: https://u-pol.com/en-us/product/coatings/specialty-coatings/reface-2k-polyester-spray-putty/

When I checked in at that stage, the stuff appeared to sand very nicely, very smooth.

[EuropaTC]

My painter used this stuff: https://u-pol.com/en-us/product/coatings/specialty-coatings/reface-2k-polyester-spray-putty/

When I checked in at that stage, the stuff appeared to sand very nicely, very smooth.

Yep, I've used Reface on both the Elan & Europa. Very thick to spray, looks awful going on but settles down nicely. Dry sand to remove any obvious dust, etc, and then prime with 2-pack is the way I would go again.

[GavinT]

Many of the visible paint cracks are only in the paint. That is when you sand to the fiberglass body (and often only to the primer layer) the crack is gone.

In your pic, apart from the broken corner, the cracks in the paint radiate from that damage. I wouldn't be assuming they aren't part of the same event.

It may well be that sanding has mostly removed surface cracks under the paint, but the sanding also tends to hide the little ones, too. Me? . . I wouldn't take the chance. But I like the method - doing the repairs with the paint initially intact so you can see the issues clearly.

I had my body sand blasted which is a great way of avoiding the mind numbing time soak associated with sanding the whole thing. It also means I might miss a few little surface cracks but it'll be covered with veil which should ameliorate that.

I stuck with polyester and chopped strand mat for compatibility reasons. Kitty hair is also marvellous stuff.
I don't think anyone would argue about the strength of epoxy but if the repairs are done properly, it's unnecessary, I reckon.
That said, Fort might chime in and point out anything I missed! 

[dakazman]

  I use upol 2251, It's been replaced by up2253.   The reface seems fine also but i wonder why cleanup is acetone and not a lacquer thinner.  It doesn't seem to have any other colors than gray whereas 2253 has 4 shades plus black. 
  I stay away from epoxy, to each there own. Veils are good and as soon as the humidity goes away down here in Florida I have a few panels to test.
 Dakazman.

[EuropaTC]

  I use upol 2251, It's been replaced by up2253.   The reface seems fine also but i wonder why cleanup is acetone and not a lacquer thinner.  It doesn't seem to have any other colors than gray whereas 2253 has 4 shades plus black. 
  I stay away from epoxy, to each there own. Veils are good and as soon as the humidity goes away down here in Florida I have a few panels to test.
 Dakazman.

I do have acetone in the workshop but for the Reface I've only ever used the cheapest cellulose thinners to clean the gun afterwards. I vaguely recall you can use acetone to thin it but I don't, just use a large jet size, gravity gun & adjust pressure until it works. (high tech, that's me)
As for epoxy resins, ditto, I don't use them either. I know they are stronger but don't see the logic in mixing resins, at least when it's all the same stuff you should get the same expansion/contraction/flexing/etc.
Brian

[Dilkris]

As for epoxy resins, ditto, I don't use them either. I know they are stronger but don't see the logic in mixing resins, at least when it's all the same stuff you should get the same expansion/contraction/flexing/etc.
Brian

 
I am on a steep learning curve and information gathering journey regarding body work but "yes" this comment makes sense to me. 

[gideon]

I'm in the epoxy camp.   The bottom line for me is that epoxy resins stick much better to everything, including old polyester resin, than polyester resins do.  I don't plan to ever use polyester resin for repairs, so I don't worry about polyester over epoxy. 

There is a huge difference between the thermal expansion coefficient (CTE) of glass and any of the polymer matrix options, so in practice the integrity of the repair depends upon an adequate fiber/resin ratio in the repair and having a good bond between old and new and between resin and fiber.  Epoxy resins deliver a much stronger bond than polyester resins, so I trust them more.

For the interested, you can bond polyester to cured epoxy if you must (though preparation is critical). See here -

https://www.epoxyworks.com/index.php/polyester-gelcoat-over-epoxy/

A source for some CTE figures

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

[dakazman]

I'm in the epoxy camp.   The bottom line for me is that epoxy resins stick much better to everything, including old polyester resin, than polyester resins do.  I don't plan to ever use polyester resin for repairs, so I don't worry about polyester over epoxy. 

There is a huge difference between the thermal expansion coefficient (CTE) of glass and any of the polymer matrix options, so in practice the integrity of the repair depends upon an adequate fiber/resin ratio in the repair and having a good bond between old and new and between resin and fiber.  Epoxy resins deliver a much stronger bond than polyester resins, so I trust them more.

For the interested, you can bond polyester to cured epoxy if you must (though preparation is critical). See here -

https://www.epoxyworks.com/index.php/polyester-gelcoat-over-epoxy/

A source for some CTE figures

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

   Very informative Gideon.
   So the repairs you do to a 1/8" panel would have a scarf cut of 6" according to the proper repair shown here. ( 1/4" Fiberglass on these cars ?)
 https://www.epoxyworks.com/index.php/west-system-epoxy-vs-polyester/#more-1582

  Then I read about the different shrinkage rates, I have seen Bondo to polyester and the results in the final paint clearly defined a repair, much like using fiberglass cloth.

 Don't get me wrong, I agree with all the strength characteristics and durability of epoxy if laid completely over the entire panel.
  Trust me, I can see every flaw I missed a year after painting.  Watch the finish move with high temps, I can only imagine what small cracks filled with water look like after freezing temps hit. a repair that doesn't have the proper is surely to crack or heave.
 (12:1 is the minimum bevel angle recommended for repairing cured composites.)
 Dakazman
 

[GavinT]

   So the repairs you do to a 1/8" panel would have a scarf cut of 6" according to the proper repair shown here. ( 1/4" Fiberglass on these cars ?)
 https://www.epoxyworks.com/index.php/west-system-epoxy-vs-polyester/#more-1582
 

That's not too different from the method described in the Europa workshop manual.
The 'Epoxyworks' example shows a 1/4" (6.3mm) panel and three layers of glass material used - must be pretty heavy stuff, and fair enough, too because they're talking about a boat. The Europa body is supposedly 0.093" (2.3mm) thick.

For me, a significant question arises re the proposed glass material to be used with epoxy. Obviously the common chopped strand mat is out as it's designed for polyester due to the specific binder added. Yet it is often said that CSM is OK to use with epoxy so I tend to be less persuaded about the promotion of a supposedly superior system when it accepts an inappropriate material.

But don't listen to me.
Here's a YouTube video of a hands on comparison between CSM using polyester and epoxy.

https://youtu.be/uLKiDa2dqq8?si=S3703XZXIyfGVEKr

However, when someone produces a carbon fibre body, we should all embrace epoxy as the appropriate material for those. 

[EuropaTC]

Thanks for posting that video Gavin and as usual with me, "every day is a school day".

I didn't know about the differences in CSM binders before, typical me, I'd just assumed CSM was just CSM, all the same. The comparison in the video shows the different resin reactions and  the guy explains it very well. You can see that yes, you can use both resin systems but there are differences in the final result and I'll admit that without a side by side comparison and his explanation I wouldn't have been able to tell one from the other seen in isolation.

Worth watching for anyone considering mixing systems.

[Dilkris]

.....Obviously the common chopped strand mat is out as it's designed for polyester due to the specific binder added. Yet it is often said that CSM is OK to use with epoxy so I tend to be less persuaded about the promotion of a supposedly superior system when it accepts an inappropriate material....

Sorry - you've lost me here with the above statement - (and I believe it to be of some importance in this discussion) - may I trouble you to reword it somehow so as I can better understand it. ? 

[EuropaTC]

.....Obviously the common chopped strand mat is out as it's designed for polyester due to the specific binder added. Yet it is often said that CSM is OK to use with epoxy so I tend to be less persuaded about the promotion of a supposedly superior system when it accepts an inappropriate material....

Sorry - you've lost me here with the above statement - (and I believe it to be of some importance in this discussion) - may I trouble you to reword it somehow so as I can better understand it. ?

If I've got it right, it means that CSM isn't all the same stuff. The glass component is, but when it's made they use an additive to bind the fibres into that sheet of matting - whether it's woven roving/CSM, veil, or whatever.

From the video it seems the additives used can be formulated to work better with a specific resin. The video shows how easily the polyester resin is absorbed into the CSM and results in what looks like a good composite. The epoxy resin used in the same CSM does work but it's clear that it takes more manual pressure to push it in and the resulting composite panel is heavier, has more resin weight and doesn't look as good IMO.  It is worth 10 minutes of your time if this topic interests you.

There's a comment that in Europe the binder/additives are powder based and work with either resin so we should be ok in the UK. But that immediately flagged up "there's a difference in CSM which could depend on where it's sourced" which made me think that if I ever wanted to use epoxy on the car then I'd need to be careful. But I'm not likely to, I don't see the point in putting a stiff area in the middle of an otherwise comparitively weaker panel.