Sika Bonding

Looks like I'm getting close to bonding the canopy on my 7 with Sika. I've read many posts and various sites regarding this process. Special thanks to Mr. Hill and Mr. Swayze for your excellent pictures and narrative.

I understand the need to finish all edges after trimming and potentially up to a 400 grit to get a nice smooth edge. My questions is: what did you use for the "scuff" for the SikaFlex to key into for bonding? I believe I've seen maroon Scotch Brite pads up to fairly heavy grit sandpaper referenced during my research. It seems something too light is not the best, while too aggressive being counter-intuitive to the fine finish after the trims are made.

Thanks!!!!

I just scuffed with a Maroon Scotchbrite pad. Clean the surface with the Sika cleaner. Then the Sika primer then goes on like liquid (very thin stuff). It provides the bridge to bond the plexi to the adhesive.

The only place I scuffed on the canopy with heavy grit paper was where the fiberglass fairing gets built up on the outside front of the canopy. This is for the epoxy and fiberglass to get a decent surface to bond. On the inside of the front canopy, I did have a Sikaflex bond (which would probably hold the canopy on by itself!).

Sounds good....thanks!

Here is something else that I have learned while speaking directly with Sika.

I had seen many posts that speak about the importance of applying the Sika within 24 hours of applying the 209D primer. When we do our canopies the first application of Sika 295UV takes about 24 hours to harden, then the spacers are removed and the final application of 295UV is applied. This can be hard to get accomplished within a 24 hour window.

Some posts talk about lightly reapplying the 205 cleaner to the 209D primer if it has been over 24 hours since it was applied. I called Sika and spoke to the rep about the issues with bonding the canopy. He stated that the 24 hour time is specified because of potential contamination issues of the 209D primer.

He stated that if the environment was climate controlled and you did not touch the 209D primer, especially with ungloved hands, the time to apply the 295UV adhesive after the 209D primer is not that critical. If there is a risk that the 209D primer may have become contaminated then a clean cloth with some of the 205 cleaner can be lightly wiped over the 209D primer to clean it.

I plan to scuff the plexiglass and the painted frame with the maroon Scotchbright pad, clean with the Sika 205, apply the 209D primer, adhere with the 295UV between the spacers, wait 24 hours, remove the spacers and apply the final fillet of the 295UV.

A couple of notes

Make sure your scuff line is right up to your tape line otherwise the primer will not stick properly and when you pull up your tape you'll get a jagged edge.

Regarding the tape line, give yourself lots of width to work with. The filet will be larger than you think an push your tape line if too close.

Do not get too concerned about the spacers. 1/16" spacers are plenty. This stuff is aggressive. Too large of spacers can mess up your overall fit. On my slider I went from 1/16" on the top to nothing on the edges of the frame. Everything is very solid and fits great.

Also note that the Sika only cures at the rate of about 1mm per day. So a thick bead my take a week to cure. And it depends on atmospheric moisture, so it could take longer in a dry climate. I left my first coat more than a week before removing the clamps.

Cure rate

tdk said:

Also note that the Sika only cures at the rate of about 1mm per day. So a thick bead my take a week to cure. And it depends on atmospheric moisture, so it could take longer in a dry climate. I left my first coat more than a week before removing the clamps.

Click to expand...

How do you know when it's fully cured?
It's really dry and the air thin at 7,500'.

There is a graph in the Sikaflex 295 UV product data sheet that shows the depth of cure at 50% RH for two temperatures. 8mm at 8 days at 10 degrees C. 10mm at 8 days at 23 degrees C. The curve gets flatter after that, because it becomes increasingly difficult for the atmospheric humidty to penetrate.

I think as long as you have enough area cured (in the thin spots) to hold the canopy in place, that it is safe to remove the clamps and apply the second layer of Sika. Wait another week, then get busy with the fiberglass, paint, etc. It will be fully cured long before you fly ;-)

You can also leave a thick test bead on piece of scrap, and push your finger into occasionally. It is pretty easy to tell when the marshmallow is still soft inside...

Humidity

We don't come close to 50% unless it rains. But, I can run a humidifier on high!