24-04: Cleaning up the wingtip transition to the wing skins

Overall, the wingtips fit great as they were. However, the gap between the fibreglass and the wing skins was not even. I decided to follow a similar process to how i finished the empennage fairings. 

I started off by sanding off the mould seams (where the parts were joined at the factory). I then sanded the moulded flange step along the leading edge portion of the wingtip and then sanded a 30 degree (roughly) bevel along the edge of the part. The idea here is to give a nice surface for the micro to stick to, and give a thick edge against the wing skins (rather than a thin, fragile feather edge). I used a Permagrit bar for the sanding - it is very course but works great on fibreglass.

Here you can see the bevel on the edge of the part, and how i thoroughly sanded the areas which will receive micro to assist it in sticking.

This shows the foreground has no bevel, and further along the part the bevel has been sanded in.

Fixing the trailing edges

The trailing edges of the wingtips extended a fair bit past the trailing edges of the ailerons and needed to be trimmed back. I used a long piece of square aluminium tubing and carefully aligned it with the aileron and carried a line over to the wingtip where it needed to be trimmed. I did most of the trimming with the Japanese saw, then placed the wingtip back in position and sanded it to the final dimension. I clamped the square tube in place and sanded down to the tube. The trailing edges were bevelled to be normal to the top surface of the aileron. 

Bad picture, but sort of shows how the trailing edges were sanded perfectly to the tube.

Prepping for Micro

To prep for the application of the micro mix and to prevent dust entering the wing internal areas when i got to the sanding portion, i taped up the end of the wing. I added some electrical tape on the end of the wing skins, and a second piece inboard, along with 2 rows of masking tape - to protect the wing skins when sanding later. 

The tape was pushed into this 90 degree angle, to try and get the sharpest, and thickest edge of micro i could along the edges of the part.

The additional tape is to protect the skins when sanding.

Applying the Micro

To make up the micro, first i made up a batch of West Systems epoxy using 205 slow hardener. I then kept adding West Systems 411 filler - aka micro-balloons or micro-glass spheres - to the mix until the mixture was thick but spreadable. What i was aiming for was a very slow seep when held vertically in a thick glob on the mixing stick. The whole mix is tipped out onto a plate to stop any exothermal reaction making it go off too fast. This was then applied to the wing using a hotel card - initially using an inboard motion toward the skins - to make sure i didn't trap any air bubbles underneath against the skin and make voids. Then again in the opposite direction to ensure a flat and thick enough application that i had something to sand off flush with the skin later on - but not too thick so the sanding is a pain. 

The micro is applied thick enough to give me something to sand back to but not so thick as to create too much work. 

Sanding it all off again!

Around 12 hours later, the epoxy had cured to the point where it was not totally rock hard, but was hard enough that it can be sanded. I find if you leave this step too long, the epoxy really goes hard and the sanding it a lot worse. I started with my 'cheese grater' rasp to take off the vast majority, then sanded using 60 grit and a small block along the transition of the micro. What i was aiming to do was sand through the micro so i had sanded it all off, where the tape was located on the wingtip - but not to sand through the tape! This would allow me to get the wingtip off and i could then sand the rest off to a nice transition with the rest of the wingtip surface. There was a lot of surface area and i didn't think the wingtips would come off! But with a bit of coercion they both did thankfully. 

This shows how far i sanded - just enough so the tape was showing all along the edge of the part but not so far as to go through the tape. This allowed me to remove the wingtip. You can see in this photo why you need the tape to protect the wing skins.

Making a consistent gap

Once the wingtip was removed, i used 60 grit to sand off the rest of the micro transition to the wingtip surface. I then removed all the tape from the wing and placed the wingtip back in position. I used 240 grit to finalise the micro thickness as it can be slightly thick in places due to the thickness of the tape. I then use a small piece of hacksaw blade to add a small 1/32" approx. paint gap between the part and the wing skins. 

The micro transition sanded to the wingtips.

Definitely need a mask here.

This shows what the edge looked like before i cut a paint gap.

A hacksaw blade was used to cut the gap..

..and gives a very nice, even gap. 

Next up, it would be time to final coat and finish the wingtips. But i realised that the wingtip light lenses don't fit that well - the thickness of the flange from the factory is not great and will need some micro seam work as well. So, next step will be to fit the landing light lenses.