Sand it back to decent wood if rapplying varnish - noone puts varnish over grey wood. If it's outdoors, I should look at the advanced woodstains such as Sadolin, they are supposed to be breathable and resist flaking. They are available in clear and natural if you don't want a strong colour.
Andy
I have a little job at a school and I have some wood that has been varnished at some point in its life that has flaked. The un-varnished wood is now grey. What should I do to prepare it ready for a new application of varnish
Dave
I'm not clear whether all the old varnish is off. If the varnish has failed in patches, and the patches are grey, it may be best to strip the old varnish off completely. If the majority is sound, and only a small section has flaked, you may get away with a patch repair.
Some ideas, depending on how fastidious you want to be!
- The best way of stripping varnish is a hot air gun and a Tungsten Carbide scraper. Sanding off varnish is a nightmare as it clogs abrasive paper
- Grey wood can be sanded (or better carefully scraped with a TC scraper and then sanded) back to bare wood. However you may be able to restore it using a proprietor wood restorer or oxalic acid (handle with care, poisonous)
- Prep the bare/restored wood with 180 grit abrasive paper. Old varnish is OK with 240 grit
- You can get a rough idea what it will look like after varnishing by wiping over with white spirit. This highlights colour variations and cross-grain abrasions.
- If you are repairing a patch, you need to feather the edge of the patch - maybe as much as an inch around the bare wood. Otherwise the sharp edge of the old varnish will show, and will also be vulnerable to failure
- A patch should be progressively built up (thin coats) just above the same level as the surrounding varnish, and then flatted back level, before the whole surface is varnished
Roland
"Roland Mann" wrote | "Dave" wrote | > I have a little job at a school | - Grey wood can be sanded (or better carefully scraped with a TC | scraper and then sanded) back to bare wood. However you may be able | to restore it using a proprietor wood restorer or oxalic acid (handle | with care, poisonous)
I can remember in 4th year at junior school we were all made to sand and varnish our own desks.
Owain
About 45% of the varnish has failed, most of it in strips :-(
snip
This sounds like the way for me to go if I get landed with the job,
Many thanks for your reply Roland, it looks like it may be a summer holiday job, so it will have to wait a bit longer before it gets touched. As you may know, schools don't have much money for this sort of work and I am expecting the head to give me the task of restoring wood.
Dave
Hi,
What sort of wood and weather conditions are there? Is it possible for water to get into it somehow if it's revarnished?
cheers, Pete.
Now that puts you 'about' the same age as me :-)
I'm 58 and still remember those days :-( Flying chalk and sometimes flying black board dusters and they were a lot heavier than the ones used today :-( One of the worst sights was the corporal punishment. Bend down and take six of the best from the biggest plimsoll that the school could find. I saw the hardest of the school take that and go away with a wet face.. He deserved it though.
Dave
Not sure of the wood, but it is earlier than the soft mahogany that the school uses now, thank gawd. One joiner described the new wood as planing straw.
I suspect that it might be mahogany, but I have no proof of this, just my colour blind eyes tell me that it might be that wood.
The wood faces South, hence the sun damage that has resulted in the varnish dropping off :-(
I can't answer your last question about water ingress, as I have only had this job for about 10 months. It is a hobby job and I want to do the best for the little children, rather than the cash rich, pound crazy council. The cash poor school need a little help here.
Many thanks for any input you might make to this problem. I do have a few gallons of Bourneseal. Is this of any use to the problem? Or is it not good enough for outside work (I have not read the label yet, so sorry for my last question about the Bourneseal)
Dave
A bad craftsmen always blames their materials ;) All the decent mahogany these days is illegally logged, ask HM The Queen. Though if it's a state school they're not really supposed to use tropical hardwoods >:(
There's a few reasons why the varnish might fail, but one way to guarantee a good result is to plane or sand the weathered wood off and give it a good coat of clear epoxy followed by the 3 or 4 of coats of good quality exterior varnish.
Another way is to plane/sand back to good wood and get hold of some stuff called "Bondaseal Clear":
Bourneseal looks like a floor sealer, and so won't be a lot of use outdoors.
cheers, Pete.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.