I think rjs raises two valid points.
First, part of the life of this wood is gone forever. The wood can never be like new again. With metal, you can sandblast the surface, re-etch and refinish, and expect a life for the finish that is as good or better than the original finish and should last as long or longer. This is not really possible with wood that has been outside in the weather, generally speaking. I would never expect a refinished exterior wood surface to last as long as the original. There are exceptions, of course. There are always exceptions. But it is important to tell the client that
a refinish will have a shorter life. This way you cover yourself. I've also learned the hard way to not make a claim for a finish based on what a paint store employee tells me. Experience is the best teacher here. Better to not make a claim for longevity unless you know for sure.
If a client asks for a guarantee, the only guarantee I would give in this case is that the repaint will not last as long as the original. In other words, no guarantee at all. Ideally, this "non-guarantee" should be in writing.
This job is a good argument for the "don't throw good money after bad" line of reasoning. But, of course, clients sometimes do not reason well.
There are many high quality, long-lasting latex finishes on the market for wood. They have fungicides, mildewcides, etc., not commonly found in oil-based enamels. The technology is better than ever. But a good job starts with a sound substrate. This substrate is surely not as sound as when new.
Declining this job is an attractive option.
Secondly, the last person to touch a job like this is the one who is blamed when it fails. How many of us have got a car back from a mechanic with a new problem that it didn't have before the mechanic touched it? It may be pure coincidence, but to whom do we assign blame? There's a certain simple logic in our head that convinces us that the mechanic is the culprit. All auto mechanics, including the good ones, run into this situation, and it's a game they can't win, because they can't see into the future. They simply lose the customer and move on.
For us sign mechanics, working on a worn out exterior wood surface is a game we can win, because we can simply decline the job. Clients don't like it, but it tells them you're serious when you say that you think the repaint will not last. Tell them, with earnest face, that your very reputation is at stake and you can't risk it. Or just say, "I just can't bring myself to waste your money." Don't say you don't want to repaint upside down Ns. Be sincere. Then let somebody else bite on this job. If someone else does the job and it begins to fail prematurely, you may get the client back as a customer, anyway. They may view you as trustworthy, somebody they should have listened to.
On the other hand, if somebody else does the job and it lasts15 years, find out how they prepped and what the heck they used for paint and go get some for yourself!
What paint to use?
The best advice about repainting wood structures would come from commercial painting contractors rather than us sign people. Painting contractors, at least the good ones, tend to know the latest information on what finishes are best. And they work with wood structures all the time--houses.
Paint for exterior wood needs to be flexible
When I was young, a house painter in Arkansas told me that wood siding moves. The grain, the pores, expand and contract with changes in temperature and humidity. So he said the best paint for wood siding is one that is flexible, that would be able to move with the wood. Latex finishes, called emulsions in many parts of the world, are flexible, stretchy, especially the ones with a high percentage of acrylic resin. Oil-based enamels that dry hard are not very flexible. Sometimes they can be made flexible for certain applications. You can put a flex additive into Matthews Polyurethane paint, and into other automotive-type paints as well, that will allow you to spray it on flexible car bumpers or vinyl sheeting, for example. Krylon Fusion, by Sherwin-Williams, had a solvent system so aggressive it practically melted itself permanently into 3M vinyl. But even though paints like these may generally adhere well to wood, when the wood is exposed to weather and the grain opens and closes, the paint tends to crack. Another painting contractor told me to think of oil-based enamels as like a hard shell. He said it's great for metal, but not good for exterior wood.
Paint for wood should not trap moisture
The Arkansas painting contractor I mentioned also said latex paint is permeable. It allows moisture to move through it, like cheesecloth. He said that wood siding did not even need to be totally dry before painting with latex, that moisture could evaporate through the finish. With oil-based enamels, the wood needs to be bone dry. A finish that creates a hard shell traps moisture inside, but the moisture will force itself through the finish to exit, creating blisters. I've seen sign installers paint moisture-laden wood signposts with One Shot that had large blisters and bubbles the very next day.
One of the reasons MDO board holds up so well with oil paint is that the paint does not contact the wood except at the edges. The wood grain still opens and closes as with any other plywood product, but the paint is insulated from it by the MDO paper. So the paint film lasts longer by riding on top of the paper layer.
About spray-painting wood structures
I often ask painting contractors if spraying is better than brush & roller for wood structures. Many say that spray is more for "down and dirty" knock-it-out work. Jobs where the budget is of concern. They have said that the highest quality finish comes the old fashioned way with brush and roller. No one ever seems to explain why, though one guy said it had to do with a "bounce effect" from the high pressure of an airless sprayer. A commercial airless has so much pressure that part of the paint can't even adhere but just bounces off, although the pressure is enough to inject paint into your flesh. I've been told you can lose a finger that way.
Yet...no painting contractor these days seems to be without an airless rig. It may be the only way they can compete and make a living.
And maybe it's the only way to do a knock-out sign job outside that may not last that long anyway.
Brad in Kansas City
