CharlieT wrote:
Regarding pore fill, I think Vijay was OK with using epoxy as long as it is sanded back to bare wood and a few coats of shellac are applied before applying the Royal-Lac post cat. I would think that should take care of the shrink back issues.
I would have thought so too, but that wasn't my experience. I still had visible pores after doing this. This post-cat formulation shrinks back more than the original.
I contacted Vijay about this. Here is his reply.
Quote:
It is a very common misconception people have with regards to shrinkage. Let’s look at it very simply; After a layer of any type of solvent based finish is applied on a substrate, the process of solvent evaporation begins. This process continues till all solvent is removed from the film. Yes, the majority of the solvent evaporates with the first few days and weeks but residual amounts remain and works its way up over a long period of time. Even shellac flakes that you buy have actually small traces of alcohol in them. They will weigh ever so slightly less after a year or more! Anyways, as the majority of the solvent evaporates, the solids molecules start to get closer to each other thus resulting in shrinkage. Therefore, all finishes will shrink based on the amount of solids it contains. But there is another parameter that is usually not taken into account; density of the solids. Synthetically engineered finishes have high density and hence tend to shrink less when compared to shellac. So if you have a finish that has 40% solids and compare it to Royal-Lac, which is also 40%, it is still not an apple to apple comparison because the density of a shellac based finish will be lower.
Solution: Two ways. Either the pore filling is done using a no shrinking material, which is not possible given the laws of physics as explained above. Or apply extra layers of finish to compensate for the shrinkage.
Pore filling: Epoxy is fine as long as it is sanded back to wood and a layer of Seal-Lac or shellac is present and acting as a barrier between the epoxy, which is alkaline and Royal-Lac which is acidic. As a matter of fact most solvent based finishes are usually acidic. Care has to be taken that there are no sand through spots. Here, wait for the shellac to dry for at least three weeks before level sanding with 600 grit and finally applying coats to fill the scratch marks of the sand paper. Let dry for a week. No sanding after this.
Extra layering: Apply extra layers of the finish and compensate for shrinkage over time. A bit of experimentation is required to know how much extra to put on. Final level sanding and buffing also reduces the overall thickness.
As I read his response, I was surprised to hear the recommendation for 3+1 week for the barrier coat. So, I replied back to get clarification. Here is his response.
Quote:
The type of pore fill does not matter. You will have to decide on what works best for you and your process. Two factors to keep in mind are:
1) Pore fill must be sanded back to wood.
2) A layer of Seal-Lac or shellac acting as a barrier coat without sand through is essential.
I had suggested that the barrier coat be allowed to dry for at least 3 weeks since most of the solvent will evaporate during that period of time. The time could be less too if the ambient temp is higher. The 3 weeks is a ball park number to start with. A barrier coat is simply as the name implies. It need not be pristine looking because the finish coats are going to sit on top anyways and the top most coat will be leveled and buffed in the end. Therefore only a very light sanding is required just so to get a level surface.
The number of extra coats is determined with experimentation. The most ideal condition is over a period of six months.
One of the attractions of Royal Lac Post Cat for me was an abbreviated spray schedule. But, if I were to follow Vijay's recommendations, the finishing phase would be very long. At this point, I'm not sure if it's worth investing more of my time in testing the product. I'll have to think this over a bit.
Login
Register
FAQ
