Reasons to Strip before Refinishing
From contributor C:
There is nothing wrong with wanting to strip down and apply a new finish rather than chance any unknowns that might occur by over-coating older finishes. That said, the types of pre-cats and post-cats differ as to whether and how long afterwards they can be successfully recoated before inter-coat adhesion becomes problematic. Usually the pre-cats have a window of being able to be sprayed that is 2-4 hours, but this depends on the amount of secondary resins and the type of catalyst in the coating. You can usually tell if the coating has a smaller or larger amount of the secondary resins by the pot-life descriptions on the tech data info. One that has a pot-life or shelf life of, say, six months or more, is weaker than one that has 3 months or less. Once this window has gone by, the surface needs to be sanded to obtain respectfully good inter-coat adhesion on any more coats applied.
These coatings act mainly like a regular nitro lacquer when first applied, and as they dry, the cross-linking of the secondary resins (normally alkyd amino resins) with the weak acid catalyst (normally a phosphorous type acid) begins to take over with its characteristics and coating enhanced properties (better solvent/water/other film enhancing qualities).
Once the coating has reached this stage, it no longer acts as the freshly applied coating did. It will not function like a true nitro lacquer in its ability to re-melt or re-liquefy and become one homogeneous indistinguishable film.
It may still be able to be dissolved by applying lacquer thinner or lacquer solvents (depending on the amount of secondary amino/alkyd resin content and catalyzation that has taken place), but unlike a true nitro lacquer, it will usually not become instantaneously liquefied in the same manner. It usually takes a minute or more to see this take place to whatever degree.
A simple test is to apply one drop of normal lacquer thinner to the horizontal surface and count to five - pick the solvent up by rubbing the tip of your finger across it. If it looks like a portion of the film has been removed by wiping the melted portion away and leaving an indentation in the surrounding surface, it is a true nitro lacquer. If not, it's most likely a cat lacquer of some kind, or a very old nitro finish that would need more time to show it.
What you wiped up with your finger will also tell you for sure if it's lacquer. As it dries it will become sticky if pressed between your finger and thumb; it will let you know that you have picked up more than the thinner alone.
You may also see that the longer the thinner is left on or reapplied, wrinkling is taking place. If it is a cat lacquer, rather then just the re-liquefying action, this happens if the coating is kept wet long enough to affect the secondary resins in the film. Of course no one wants this to happen, because it would mean having to strip and redo the finish, and it usually will not happen if the coating being applied over it has limited slow evaporating solvents within the solvent blend. That's why it is not seen very often.
Of post-cats, the same chemistry is present but in a much higher degree - more secondary resins and a stronger catalyst (usually the sulphonic type). The conversion process from lacquer status to alkyd amino status is much stronger and quicker and leaves virtually no real degree of reversal by lacquer solvent action once fully cured.
The fine points can be argued among chemists, but this is really the bottom line for the end users. Your competitor is not totally wrong, but by recoating older finishes, he's taking on a higher degree of liability than someone who does the job the correct way, stripping and applying new material with known results.
Older finishes have in common this: most of their service life has already been used up, and a new fresh finish over them cannot take away this fact. The new finish over the old one in no way revives the old finish, and the new will forever be fighting a timeline it cannot win. Eventually the weaker old coating underneath it will fail, and then the new will fail earlier than it would have, had it been done correctly from the start.
Still, it is always proper to check with the individual makers of the products you're using to know all the ins and outs.
From contributor R:
I agree with all that contributor C said. Starting a large refinishing job tomorrow that I'm stripping and was surprised at how many people told me to just go over the existing finish.(Will not do it!) It's easily scratched off with my fingernail everywhere and I just wouldn't risk it.
From contributor M:
I'm not a chemist but I agree with what contributor C wrote above. Here is another consideration: if you have a refinishing job that you can even contemplate not stripping, i.e. existing finish appears sound, and very few damaged areas that can all be matched and touched up to a satisfactory degree, you might consider sealing off the old finish with a (dewaxed obviously) shellac prior to applying your finish product. I thoroughly wipe off the old surfaces with mineral spirits and scuff up with a grey pad. I then make my repairs, and unless I've used any alcohol-based tints or dyes, I give it two good coats of a freshly mixed shellac. I will then scuff that after a couple of hours and proceed with the chosen finish schedule. Shellac is a great barrier layer - it adheres well to almost any existing finished surface (if properly cleaned and scuffed), and almost any other finish product will adhere to it. Just a suggestion that has worked for me in the past, especially when the price a customer wants to pay for a refinish job doesn't cover a full strip! Obviously if the existing finish has completely failed or is in such major disrepair that it can not be satisfactorily repaired, this approach isn't relevant.
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