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Old 11-07-2020, 14:10   #1
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Join Date: Jan 2007
Location: On board
Boat: Tom Colvin Gazelle 42ft
Posts: 252
Varnishing with two part polyurethane, a warning

I posted yesterday with a question re the flexibility or lack thereof of two part paints and while doing so at one point digressed into the subject of varnishing with two part clear polyurethane. We all know how durable a finish can be obtained with this polymer but I venture to guess that most of us would also agree that tung oil varnish has a richer appearance. For us however the durability, gloss and ability to show off the beauty of wood in a tropical environment without a lot of work makes us into lovers of the two part chemical.

However that is not what i want to write about, after all why preach to the converted? Writing about this subject yesterday albeit in a different context reminded me to share about something we learned the hard way. So what I do want to warn about is an issue we had with varnishing with 2 part polyurethane varnish.

I worked for many years, all my life in fact, as a chemical research technologist and as such have an obsession about measuring things precisely. To a fault probably. I understand why one needs to follow the directions when mixing two chemicals in a polymerisation reaction. If you are not precise both chemicals will have left over reaction sites and this will affect the end properties of the polymer such as hardness, gloss and durability.

And because the measurement and mixing of those chemicals from the awkward containers they are sold in is such a pain particularly for small amounts, we started using syringes. These syringes are available from one cc to fifty and are wonderful for precisely dispensing the correct volumes no matter the volumes required.. One other precise means of measuring the right ratios is by weight and we have small scales on board for that purpose. But because one needs to know the specific gravity of the various chemicals this is not always feasible.

In any case we decided to use the syringes and we also chose to use a much cheaper version of a clear two part polyurethane than the expensive Interlux Perfection Plus we had previously used extensively on the interior wood of GAIA. We were in Malaysia at the time and I do not remember the name of this product but I do remember it being one half of the cost.

We immediately ran into trouble. Our first coat was badly 'cratered' or 'fish eyed' and we immediately suspected surface contamination but no matter what we did to prepare the surface differently fish eyes continued to appear. Fish eyes are awful things, well in varnish anyways. They force you to remove all the newly applied varnish by sanding and start all over again. We then looked at and changed the containers we used to mix the varnish in and ended up only using small stainless beakers. But to no avail.

We then broke down and bought a quart of the expensive but familiar Perfection Plus and guess what? Fish eyes all over our beautiful cherry wood!

In the end after a lot of time consuming sleuthing we found that without a doubt the problem came from our syringes. It would appear that the rubber seals made of I do not know what contaminated both polyurethane products so badly that they, the syringes, could not be used.

So do not be tempted. Do not use syringes for dispensing either part of two part polyurethane.

I hope this heads up will help someone avoid a lot of time, work, effort and expense.

Happy varnishing

Jim SV GAIA btw we in the last few days had occasion to open our cans of Perfection Plus as we had used all of the much cheaper product, and after several years of sitting well closed on our always in the tropics boat, it was fine to use and the first couple of coats measured out with spoons, look great.
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polyurethane, varnish

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