Archive for the ‘finishing’ Category

Un-sticky Glue

Friday, March 29th, 2019

Because the veneer on commercial grade plywood is pretty thin, I tend to be extra careful with glue around it. I don’t want to have to do any sanding because glue made its way onto the thin veneer. When edge banding strips of solid wood onto the edge of plywood it’s almost impossible to not get glue onto the veneer surface.

One way to deal with this problem is to quickly wipe away the glue squeeze-out with a damp rag and another is to prevent the glue from sticking and deal with it after it dries; I prefer the latter. Several years ago (~2008) I bought a can of Waxilit, which is a very soft wax. Applying the wax to the surface of wood prevents glue from sticking and after the glue has dried onto the wax surface it is easily scraped away with your fingernail or a piece of plastic. After all of the glue is scraped away, the wax can be removed by wiping the area with a rag dampened with mineral spirits.

A while back I ran out of Waxilit and recently I decided to try ordinary paste wax (I use Johnson’s paste wax) and the results were the same as with Waxilit. I apply the paste wax near the edge of the plywood where I will be gluing on the edge banding. After scraping away any dried glue, the paste wax is easily removed with a rag dampened with mineral spirits. And if you are using a wipe on oil-based finish, any residual wax that is not removed will dissolve into the finish as it’s applied. This is because wax, mineral spirits, and oil-based finishes are all nonpolar or lipophilic (lipid-like or literally oil loving) substances and if you remember from your high school chemistry class, ‘like dissolves like’. This means that nonpolar substances, like mineral spirits and oil-based finishes, will dissolve other nonpolar materials.

Give this technique a try on scrap wood and see if you’re happy with the results; I think you will be.

How does “Polyurethane” work?

Tuesday, September 8th, 2015

(**Every attempt was made to make this easy to read by the non-chemist woodworker.  My fellow chemists please keep this in mind.)

*Images enlarge when clicked*

 

Polyurethane is a polymer (a long chain molecule with repeating units) made by combining a dual isocyanate (this is just the group R-C=N=O, a very reactive species) with a poly (more than one – this is where the POLY in polyurethane comes from) alcohol.

urethane

The straight arrows on the left and right (above) indicate where the polymer could continue on, resulting in a larger polymer.  So, a polyurethane is a long chain polymer of an isocyanate and a polyol (alcohol with more than one -OH).  As soon as the two are mixed the reaction will start.  So, as soon as they are mixed the curing process begins.  This isn’t very useful for a woodworker (some professions might use a two part polyurethane).  If the manufacturer mixed them in a can and sold them to you, by the time you opened the can it would be a solid.

Another way to make a film finish is to have everything in one part with the curing process a little slower and controllable.  This can be achieved with a uralkyd finish.  A uralkyd finish is really what you’re most likely buying at the home center when you buy “polyurethane”.  A uralkyd finish combines features of a urethane (above) and an alkyd.  What the heck is an alkyd?

An alkyd is a polyester with fatty acids sticking off of it.  The fatty acids are what give an alkyd finish the ability to polymerize (with oxygen).  The process is the same one that gives drying oils, like tung oil, their ability to cure in the presence of oxygen.

Below is a uralkyd resin that I drew.  The resin below is only an example and the version of uralkyd that you are using will be different (the differences are going to be proprietary by the company that sells it) but the principle is the same.  A uralkyd can be viewed as a fatty acid modified polyurethane.

uralkyd3

After a uralkyd finish is applied to your project and the solvent evaporates, it will begin to cure.  This happens at the double bonds  (shown with arrows above) which react with a molecule of oxygen (below).  After this, it will react with another uralkyd molecule (the R-OOH will first break down into R-O radical and OH radical, but don’t worry about this – see fat arrow below).  The process of the double bonds reacting with oxygen, and cross linking, can be accelerated with metal driers which is usually added by the manufacturer.  I might dive into the effect of metal driers in more detail in another posting (depends on how this one goes down).  The section of the uralkyd shown in red below is linoleic acid.  Linoleic acid is a component in a lot of drying oils, like tung oil.

polymerization2

Neat huh?   Please let me know if you have questions.

 

** EDIT (9/9/2015) a uralkyd is still considered a polyurethane because it has poly (many) urethane (also called carbamate)  linkages.

 

Polarize Your Projects

Wednesday, July 2nd, 2014

Recently I have been trying to learn more about photography. I figured this would benefit my woodworking hobby by allowing me to, well, you know, take better pictures of my projects. I purchased a Hoya HD polarizer and I started playing around with it today. Wow! The effects are dramatic. I will definitely be using this filter to reduce glare on finished pieces and bring out more depth. Normally I wouldn’t shoot a picture with such a large glare spot, but I wanted to highlight the effect.  Most regions of the table top look improved; not just under the glare spot.  Pre and post filtering pictures below.

Click to enlarge photos.

pre1

post