polyurethane finishes

Hi all,

I just purchased a glider with a factory PU finish. I'd like to know more about them, including:

- best practices for maintenance
- failure modes
- method for repairing minor dings etc.
- causes of wear and tear

I am pretty familiar with gelcoat but know very little about PU.

Thanks,
Matt

On Sunday, April 18, 2021 at 9:48:01 AM UTC-7, wrote:
Hi all,

I just purchased a glider with a factory PU finish. I'd like to know more about them, including:

- best practices for maintenance
- failure modes
- method for repairing minor dings etc.
- causes of wear and tear

I am pretty familiar with gelcoat but know very little about PU.

Thanks,
Mat

Hi Matt,
First off, there are lots of urethanes available, but the one used by factory is Acrylic Urethane and it is an unbelievable great finish............with one notable exception! It will blister if left in an closed trailer too long! My bird is 20 years old and the finish looks as good today as the day it was produced, but it spends its Winters inside its trailer which is inside my barn! If you are unable to get inside storage, I’d recommend opening your trailer every month, even if you don’t fly, let the damp trailer dry out!
As for upkeep, I wax her every Spring and buff and wax it every other year.........changing the transition tape after buffing, but before new wax is applied.
AU remains ever so slightly flexible and won’t crack. Scrapes can be filled, sanded, primed and painted in a conventional manner. Color match can be a problem, but several manufacturers use an auto paint color, so exact match can be ordered...........Most will not use any “tint�, so specify “no tint� in most cases.
Hope this helps,
JJ

On Sunday, April 18, 2021 at 7:53:41 PM UTC-4, John Sinclair wrote:
On Sunday, April 18, 2021 at 9:48:01 AM UTC-7,
Hi Matt,
First off, there are lots of urethanes available, but the one used by factory is Acrylic Urethane and it is an unbelievable great finish............with one notable exception! It will blister if left in an closed trailer too long! My bird is 20 years old and the finish looks as good today as the day it was produced, but it spends its Winters inside its trailer which is inside my barn! If you are unable to get inside storage, I’d recommend opening your trailer every month, even if you don’t fly, let the damp trailer dry out!
As for upkeep, I wax her every Spring and buff and wax it every other year........changing the transition tape after buffing, but before new wax is applied.
AU remains ever so slightly flexible and won’t crack. Scrapes can be filled, sanded, primed and painted in a conventional manner. Color match can be a problem, but several manufacturers use an auto paint color, so exact match can be ordered...........Most will not use any “tint�, so specify “no tint� in most cases.
Hope this helps,
JJ
The blistering is so irritating. A friend refinished his Phoebus in AU and experienced blisters in area of the fuselage dolly. We suspect that was caused by water leaking through a seam in his trailer roof getting the dolly wet. This happened is a short time during the summer since he stored the glider out of the trailer in a heated garage during the winter. I have also seen blistering on an SZD Junior horizontal stabilizer trailing edge (not the elevator), seeming from moisture entering the sharp edge ( never in a trailer but always in a hangar.) There are also many accounts of cars being refinished in urethanes and experiencing blisters while in storage under car covers, even in barns, in some cases fairly soon after painting and in other cases 20 years later! Just when we thought urethanes were the answer! I suspect in all cases it comes down to proper application.

This sounds good! I will make sure the plane stays dry. It is 18 years old and looks brand new. It's been stored in a trailer in a garage and has not flown much. I will keep it assembled in a t-hangar where water should not be an issue.

Urethane finishes are less prone to cracking and do not change their color over the years. They are in general more durable than Polyester. But cracks in the underlying laminate are more difficult to spot. In case of repairs, you will need to make sure the surface is flat before applying the final coat. Colors can be matched more easily than with T35 and small repairs are easy to do. Sanding requires high quality materials, but they easy to polish unless they contain too much softening agent. An urethane finish definitely worth the extra money when buying a new glider.

Blistering occurs with Polyester and Urethane finishes if the surface has been wet-sanded and not dried properly before the final coat is applied.

Christoph

On Monday, April 19, 2021 at 10:58:39 AM UTC-4, Christoph Barniske wrote:
Urethane finishes are less prone to cracking and do not change their color over the years. They are in general more durable than Polyester. But cracks in the underlying laminate are more difficult to spot. In case of repairs, you will need to make sure the surface is flat before applying the final coat. Colors can be matched more easily than with T35 and small repairs are easy to do. Sanding requires high quality materials, but they easy to polish unless they contain too much softening agent. An urethane finish definitely worth the extra money when buying a new glider.

Blistering occurs with Polyester and Urethane finishes if the surface has been wet-sanded and not dried properly before the final coat is applied.

Christoph
You say that with some sense of authority but I have seen urethanes blister when they either were not wet sanded at all or only wet sanded on the final coat. I’m also sure the SZD factory knows what they are doing and yet that one is blistering too!

The process details and products on this page

http://www.craggyaero.com/ventus.htm


Held up extremely well on my wings done 20 years ago. The glider was out in the sun from April to Oct in Northern California and in a trailer outside all winter. No blister etc.
The page above details the refinish of the fuselage, it was also done 20 years ago with PolyFiber waterborne polyurethane that did not hold up at all.
The acrylic urethane used on the wings and the refinish of the fuselage was PPG JAU Direct Gloss Acrylic Urethane Shopline, 1/2 the price as the expensive stuff.

Richard
www.craggyaero.com