Hi guys,

I'm in the process of working on my first composite project  which involves skinning an ABS plastic car spoiler. I'm about midway through the project and wanted to get some advice for the subsequent steps as I've come across something strange.  
Forthe project I used some 200g plain weave CF fabric and some 3:1 high strength epoxy resin that is supposedly meant for CF/kevlar fabrics.  
After abraiding the surface with 80 grit sand paper and keying the edges, I put a coat of well mixed and accurately measured epoxy. Once at the b stage about, 1h45min later, I laid the CF fabric and teimmed the excess. Another 3 hours later a second heavier coat of epoxy was applied. All work occurred in a clear environment which was at a constant 20-21C. The results looked promising at this stage:



Once cured, about 5 days later, I re sanded with 180grit and 80grit to take down the low spots and used a dremel to take off the hardened overhang. I now applied another 5 coats, of epoxy, where each coat was applied to the previous at the b stage, about 1h40min. 

It is at this stage I noticed the epoxy starting to turn milky near the surface, in more of a streaky fashion. This was most visible by the 4th coat and 5th coat. I left the spoiler to cure for about 30 hours. The epoxy did harden and looked like this:


Suspecting the streaking was some form of amine blush, albeit the surface was hard and I could not wipe off the streaks, I proceeded to wash the part with warm soapy water and a rough sponge. Once I dried the surface the epoxy had turned white with the same streaking patterns becoming more pronounced. Luckily I only washed a portion of the spoiler, however the more I tried washing off the supposed amine blush the white the piece became. It is worth mentioning that the epoxy in question is a non blushing type. 

After initial washing:

After additional washing:

Worth mentioning, when the part is wet the epoxy shows clear and the weave is nicely visible. It is only when dried that the part returns to the whitish color.

At this stage, I'm at a bit of a loss as to what I've come across. I still need to block sand the part to remove uneaveness in the part. I was planning to add a final coat of epoxy and then do a final polishing but was hoping to get some advice from the community first. Any thoughts or comments on how to proceed are welcome.

Thanks for your time guys!

Telram
Montreal, Canada

The white is the effect of the moisture from either sanding too early or a slight issue with the mixing ratio (and hence the blush you had).  In either case, the resin is left hydroscopic and hence wet sanding causes the white issues you have seen.

I would flat it back until the surface is smooth  and hopefully you will have cut through any  bad resin and be down to good stuff.  You can then coat over the top to give a final sealing coat.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

So, I'm pretty much at my wits end.

It sanded down the part with some 80 grit and got the part pretty smooth . Whenever I wetted a part to clean it off the fabric would shine though nicely. Things were looking good.

On the advice on a friend, I left it out in the sun for a day to help any uncured resin fully cure. The next day, the party developed some blotchiness mainly at the surface but beneath as well. 


When the part is moistened, it looks like this:


I'm really getting fed up as I've put so many hours into this part and can't get any decent results.

Any thoughts on what went wrong?

Thanks guys
T

Looks like you still need to sand through a bad layer of resin - the rings of white show where you have gone through one layer to the next.

Multiple layers of resin applied at each B stage should not cause any problems on their own.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

any update?  

Hey guys 

Sorry for the delay, I've been a little down from the results and haven't worked on this as quickly as I would have liked to.

That said, here is an two topic update:

1) So I kept sanding through to get through more of what seems to be a bad layer of resin. In some area the top layer seems to be scaling off, and the  boundary between layers seems to be apparent. 


I'm not sure how aggressive I should be with the top layer, on close inspection post sanding, the snow is still very visible. But when wet the snow disappears.




Also, when I run my finger vigourously against the wet resin, the resin seems to roll up like dead skin would.


At this point, I'm wondering if I keep sanding and take more of the top layer of resin off or if I go ahead and top coat with a fresh coat of resin. Part of me is worried that I will hit the CF later.

Thoughts?

2) After I left the piece in the sun for a day, the CF fabric pulled away from the spoiler in 3 bubbles. I can actually push the bubbles in to force them back against the ABS plastic, but they just pop back up. I've sanded around them and the tops now have small holes. 


I'm wondering if I should just pour some resin in there and just support the now bubbled fabric in place. What do you guys think?

Thanks
T

+x

 

Telram - 8/31/2018 11:51:07 PM

Hey guys 

Sorry for the delay, I've been a little down from the results and haven't worked on this as quickly as I would have liked to.

That said, here is an two topic update:

1) So I kept sanding through to get through more of what seems to be a bad layer of resin. In some area the top layer seems to be scaling off, and the  boundary between layers seems to be apparent. 


I'm not sure how aggressive I should be with the top layer, on close inspection post sanding, the snow is still very visible. But when wet the snow disappears.




Also, when I run my finger vigourously against the wet resin, the resin seems to roll up like dead skin would.


At this point, I'm wondering if I keep sanding and take more of the top layer of resin off or if I go ahead and top coat with a fresh coat of resin. Part of me is worried that I will hit the CF later.

Thoughts?

2) After I left the piece in the sun for a day, the CF fabric pulled away from the spoiler in 3 bubbles. I can actually push the bubbles in to force them back against the ABS plastic, but they just pop back up. I've sanded around them and the tops now have small holes. 


I'm wondering if I should just pour some resin in there and just support the now bubbled fabric in place. What do you guys think?

Thanks
T

Hi. I’ve don’t have the experience that others on here have but I think I would cut my losses and sand back the C/F bubbles to the ABS. If it’s not stuck at this stage it never will. Then fill and sand the resultant low area and start all over again taking great care to have a clean and grease free surface. Mix your resin very thoroughly, the bubbles could be unmixed resin. ( try the two pot method to ensure the mix is free of any unmixed pockets of resin or hardener in the corners.

Been there, produced the scrap, got the T shirt. Best of luck mate. Keep at it. It’s worth all the effort when the result matches the expectation and remember that if it was easy anybody could do it.

I agree start again.

I have never had any luck getting rid of the moisture whitening, temperatures during cure must be dry and 20C +.

To help things stay stuck you could drill many 1-2mm holes through the plastic to provide and real key for the resin to grab and prevent any releasing.

Personally I don't skin plastic parts, usually the resins will not stick anyway and you will also be fighting against the thermal issues of the plastic and carbon. Most plastics expand considerably with heat wheres carbon fibre will not expand at all so any heat will always be tempting this to release.

Welcome to the world of composites.