Twisted: Week 66 - Ducts Part 2

Hours to date: 652.25

What a mission these ducts are turning out to be.

But not mission impossible.

And I'm learning a bit along the way.

I figured out what I did wrong when using body filler on the foam plugs.

I thought I would miss out the micro layer stage - as you have to wait 24 hours for the micro to get hard for sanding, and it's harder to sand than filler.

This was my mistake.

The micro actually etches into the foam, whereas the body filler tends to sit on the surface and so when you are sanding it can grab and tear the foam. Also I was using 40 grit sandpaper to start with, which was too coarse really.

Lesson learned is - use micro on foam - at least for that first layer to sand.

I put plenty of layers of primer on once things were smooth with the body filler - this was several stages of sanding then priming again to fill in the little inevitable pinholes that appear.

After that a final sand with very fine paper and then it was onto the cutting compound and polish stage. Followed by 3 layers of release wax (3 layers is not enough as you will see later on!)

I have read that you should use 10 layers of wax with Polyester but I was planning to use PVA release instead.

I gave that a go and it didn't work - as you can see from the below photo it beaded on the wax and didn't form a smooth layer.

I don't have any spray gear in my workshop but had been given the tip that you can apply PVA okay with a foam brush (see photo below).

Maybe I shouldn't have used any wax at all before trying the PVA - I'm still not quite sure what went wrong here.

After cleaning off the PVA I was getting impatient so decided to go for it and get on with laying this thing up. (another mistake!)

So out came the gel coat and I struggled to apply it in a smooth layer - but put it on extra thick anyway.

It sets very quickly so I didn't have much working time.

Polyester is a different animal. I felt much less in control than I do when working with Epoxy.

I also experienced Exotherm for the first time. I've read about it before, but to see it first hand was a little scary to start with.

Exotherm is the reaction of Polyester resin with it's catalyst - even a small amount gets warm after 20 minutes or so. The first batch of Gel Coat I mixed was way too much and when it turned to jelly after about 15mins I set it on the floor. After about 30 minutes I went back to it and it was disturbingly hot - I almost couldn't touch it. So I took it outside to cool off. It was also rock hard by that point.

No harm done but something to be aware of.

Also it means that you shouldn't layup more than 2 layers in one session with polyester as the heat will distort the mould/plug and shrink everything - not good.

So I ended up putting the gel coat on and then one layer of fiberglass in the first session.

I left that for 24 hours then did two further layers of fiberglass. One of the nice things about Polyester is that you don't have to key up the surface before using it - so long as you are applying it on top of existing layers.

I also put some carbon on the flanges for the nut plates that are to be attached later.

Not too pleased with how this all went - I felt for sure I had screwed the whole thing up and was not looking forward to having to start again with new plugs.

So a couple of nights ago I crept into the garage and began to see if I could salvage this job.

The 3 coats of wax did not allow for easy release of the plug from the part - although it did release in the end.

Turns out Gel Coat is pretty strong stuff and if you are careful you can use a flat blade screwdriver to slide along under the plug and gouge away at things to get the foam out. Gel coat can be cut and polished later to take any small scratches out.

It took about 2 hours to get the first plug done. I've still got to do the other plug and then think about the flanges for the other ends...

Joy!