Question
WOODWEB Member:
Against my better judgment I have been ordered to follow this finishing schedule. I am hoping to have my employer browse through the comments you leave. Here's the story.
Large, solid 6 panel mahogany exterior door. Lots of casing as well as a fancy upper window frame. Two large exterior frame and panel end panels with leaded glass inserts that are to be installed on each side of the door. This door with all its parts and pieces is very intricate, with applied moldings and fancy details.
This is the finish schedule I am forced to follow...
(1) Completely fill all pores with cherry wood putty… Yes! I said wood putty, not paste wood filler or a level sealer; but putty.
(2) When putty is dry the raw wood is sanded with 150-220 and final sand with 320. Remove dust etc.
(3) Apply a dark mahogany gelled stain.
(4) Apply 9 coats of MacLac sanding sealer sanding each coat with 220 and 320 paper in between coats.
(5) When grain is completely filled, apply an automotive satin polyurethane catalyzed 4 parts to 1 part and thinned out with Home Depot lacquer thinner.
(6) When poly has dried, sand with 220 and 320 and coat again.
(7) Repeat step 6 until finished.
I am fearful this finish will degrade shortly. Right now if you even gently tap on the surface or accidently nick an edge while blowing it off, the finish chips all the way through to the wood. Also, with all the coatings that have been applied, the finish is starting to have a blue cast.
For what it's worth, the counterperson at the paint store told the shop owner that this was the way to go. He has already completed a few parts of the job (they are all wrapped up and stored away in the shop), so he has boldly told me to complete the rest of the job in the same manner as the completed parts were finished.
Forum Responses
(Finishing Forum)
From contributor M:
Why 9 coats of sealer?
From day one I have expressed my concern over the products being used on this project but to no avail. All I hear from the shop owner is that it's taking too long to complete and he wants to know when it will be done so he can install it. I am aware of the various adhesion tests but right now it's impossible to take the time to fabricate a new sample. Every working hour is spent on the completion of this project.
I will direct the shop owner to this site to read your input.
Many in finishing, or woodworking in general, believe that there are secrets and mysteries counter to logic and science that need to be obeyed for some ancient reason. This is superstition, and will always cause problems. Outside of the shop, it causes war, starvation and numerable problems.
While we cannot effect large scale change in the world, we can do so in the woodshop. Many woodworkers have found themselves in your position and this has led to them starting their own shop. If this is your cue, be sure to separate superstition and belief from science and accurate observation.
For exterior wood, all the finish ingredients must be rated for exterior use and be properly applied. Some exterior finishes are much better than others. When clear finishes are used on exterior wood, the typical good quality exterior finish requires annual maintenance. Lower quality products will fail in a few months. You will be lucky if your finish lasts a few months and it will be a nightmare to replace. Notice I said replace, not repair.
Wood putty is more difficult to use as a pore filler than the products made for that purpose. Paste wood pore filler is a better choice.
Sanding the wood surface to 320 grit is too fine and the clear coat finish will not adhere as well as it would if you sand the wood to 120 or 150 before staining.
Good quality gel stains come in handy for entry level finishers on woods that are prone to blotching (uneven coloring) when stain is applied. These woods include pine, alder, aspen, etc. Mahogany stains very well and a gel stain is not required. It would be much easier to use a spray and wipe stain.
This sealer is not rated for exterior use and will fail. Also, never use more than 2 coats of sealer (when you use it). Only use sealers that are compatible with the topcoats you're using. MacLac sanding sealer should never be used with 2K PU (2 component polyurethane). Do not use this sealer in an exterior finish.
The ratio of catalyst to finish is product dependent so follow the directions for the brand you use. Lacquer thinner is not the proper reducer for 2K PU… Use the same brand thinner as the finish and you won’t run into trouble. Note, an acrylic 2K PU can be a good product for an exterior clear coat wood finish if you choose the proper product and apply it correctly. I’ll recommend Matthews satin MAP based on firsthand experience.
Follow the recoat directions for the product you choose.
How will you know you’re finished?
For a published reference, have your employer get a copy of Bob Flexner's book "Understanding Wood Finishing." It goes into much greater detail about why most of the steps in your finish schedule are a bad idea and destined for failure. Please provide the finish schedule you propose to use including the brands and products you recommend.
As an employee one of my greatest responsibilities is to look after the welfare of my employer. I have gone well above and beyond this responsibility by expressing my objections about this finish schedule and the products being used. My objections are being ignored and I feel sad for the owner who will have to re-do this disaster sooner than later.
I am aware that exterior finishes require a maintenance schedule. This system has, however, been sold as the ultimate of finishes and I know in my heart it is not anywhere near that.
I used Imron on some structural steel for a boat many years ago. I think it was marketed as a marine grade finish and not the same formulation as their normal 2k automotive stuff? It's been a long time since I used it, so I may be wrong.
Personally I have seen 2k type automotive finishes fail when used on exterior wood work. So call it a myth all you want - but there is nothing that will outperform a properly applied and well maintained exterior marine grade spar varnish over the long haul.
When a typical varnish job gets dinged, the exposed wood quickly changes color, the water gets under the film and starts to peel it. Which results in another distinct area of some protection. This results in these aura spots when you scuff sand them. The Imron never seems to peel.
The parts I'm talking about are 1.5" x 1.5" x 10' trapezoid (triangle with the top cut off). They prevent people from slipping off the bow of a sailboat that has smooth shear. People are constantly stepping on them, lines chaff them, and spinnaker poles are dropped on them. Hell of a lot of abuse.
Many people lay down a couple of coats of epoxy then varnish over it in situations like cabin soles (teak/holly ply). The epoxy prevents damage to the wood when the varnish fails.
