When To Sand Wood Surfaces, and Why !

[CedarEd]

You might wish to improve the performance of any wood surface so it holds a stain or paint better. The USDA Forest Products Laboratory (FPL) (www.FPL.FS.FED.US) tells us that sanding a smooth planed wood surface with medium grit paper (60 to 80) is better than simply coating the planed surface, which is nonetheless still acceptable. The FPL suggests that when priming siding the rough sawn face be used as it has greater surface area and thus will hold more primer than the smooth surface. For those using a semi-transparent stain that may produce the wrong 'look' if the client wishes a smooth face siding on their home. In that case the sidings coat of semi-transparent stain can look 'blotchy' if applied to only the smooth planed wood because the natural variation in the density of any wood along the length of the siding plank may vary causing more absorption in one area and less in another. The sanding of that smooth face will result then in 2 beneifts, one will be a more uniform appearance, and second more pigment on the surface which lengthens the time before the semi-transparent stain will require reapplication. This is why many machine coaters require a signed release before simply staining a smooth wood siding or decking that may end up looking blotchy or seem to fail too soon.

Because even a small exposure to sunlight can damage the wood surface sufficiently to cause it to eventually erode, Scuff sanding is recommended if the wood has been exposed for 1 to 8 weeks. After that the surface becomes damaged sufficiently to cause relatively quick paint failures. 16 weeks of sunlight caused cracking and peeling in paint films after just 3 years, where the same paint applied to a new undamaged wood surface did NOT crack or Peel in 20 years +. Sanding removes the damaged surface, but is far more difficult if the surface has been severely weathered because it becomes nearly impossibl;e to sand all the cracks and crevices in wood joints etc. This is why machine priming of wood before delivering the wood to the jobsite has such benefit.

Washing wood with strong chemicals like chlorine bleach that has not been adequately diluted, or strong cleaners like de-greasers or paint strippers causes the wood surface similar damage and the resulting erosion that causes the coatings to peel or crack. Neutralizing such cleaners that soak into the wood is nearly impossible, so afterwards the sanding of the damaged surface is advisable to remove as much of the residue as possible before applying the new coatings, and restore smooth surface from any grain raised caused by the wetting of the wood.

After a wood surface has dried the wetting of that surface causes the grain to raise, and again sanding the surface is the way to restore the smooth appearance of the original wood. Some woods have extractives that are soluble in water, like western red cedar, rewood, cypress, oak etc. so wetting these woods can cause water stains from extractive bleed which can interfere with coating adhesion, so sanding and not wetting such wood is always the better method.

So for the best performance of the wood and the coating, store it away from dirt, water and sunlight until coated, sand the surface if any of the conditions require it, like those I just enumerated above, and install the wood according to the industry instructions, which are usually available FREE for the asking.

[Bill Robinson]

OK I 'll bite.

I am here in New Orleans where the predominate home is a weatherboard or drop siding.

Many of these homes have not been painted for several years.

The most common prep is to powerwash these homes prior to re-paint.

I do not know what solvents or surfactants are used nor do I know what dry time or paint is used.

My concern is that these homes are covered in lead-based paint.
AND that power washing spreads the lead contaminated paint all around.

AND that the lead-tainted paint, if sound, is better at protecting the wood than the new stuff.

And yes, much of these homes the wood has been exposed to the weather uncoated for some time.

So whadda' ya' have for me here?

[NW Architect]

Probably important to the equation - most of those older NOLA homes are sided with old growth Cypress siding

[CedarEd]

The lead paint is dangerous, and should be handled as the dangerous substance it is. I can see replacing it, especially if it is down to bare weathered wood, with new siding. I would not sand or power wash such siding. If the siding is intact, and paint is not cracked and peeling, it could be recoated once cleaned. No Power Washing in such a case, only cleaning with a soft brush, and mild cleaners. Remove dirt & mildew, no paint, they usually wear 'space suits' to remove lead paint these days, and the process is one most places require special licenses to allow it to be done.

Lead based paint is brittle, unlike modern acrylics. Acrylics have twice the ability to move as the wood swells & shrinks than the oil based paints, so the new paints are vastly better, which is what you saw in the tests at the FPL, paint uncracked and unpeeled that was 25+ years on a south facing test fence. Had the old siding been coated with the modern paints, and coated before the wood was weathered, you would have no problem.

If the siding is bare, as in no paint on it, you can leave it to weather, and never coat it, you can stain it with a semi-transparent stain that will need to be applied on a regular schedule, but you can never paint the siding. The severe weathering it has already been subjected to pretty much guarantees that anything you apply will crack and peel because of the sunlight damaged surface. hence it is ok to refinish but only with coatings designed to be renewed regularly. Removing lead based paint would be so prohibitively expensive, if it could even be legally done, you would probably reside the building for less cost anyway.

[Bill Robinson]

Thanks, I am familiar with the lead paint issue, see here.

I am working on a method to recommend for re-coating these homes with something that will work.

Previously painters used a power washer to remove the loose and flaky paint as well as the bio organic growth (BOG), mold and mildew. EPA lead safe work practices (LSWP) require the water from power washing to be filtered and flushed. This has caused the compliant painters to actually collect the water and store it in barrels to be disposed of later by filtering the water and flushing the filtered water.

I am in favor of scraping and sanding the loose paint using LSWP and then using minimal scrubbing to remove the BOG.

Then we get to the re coating, you are recommending a stain? Solid body stain?

As NWA pointed out the weatherboards are old growth cypress, the reside option is either new growth cypress which does not have the durability of the old growth and typically fails in a couple of years or fiber cement. Neither of which is palatable to the pocket book, aesthetic or HDLC.

Not being a painter I am not familiar with solid body stains which would look good over existing siding with some paint intact.

BTW, we are doing a project next year using western red cedar, will post photos on that as it progresses.

[No faux zone]

Bill,

The Architects I work with on Historic projects with the same type of issues always spec SW duration for this type of work. Usually preped by hand scraping and sanding the edges of the remaining (and well adhered) existing paint so it has a feathered edge. Work very well at filling in fine "alligator" type cracks too.

I went to a job we did about 5 years ago to look at some other work and the paint has held up very well. I believe we used a SW oil based primer for the bare wood.

[CedarEd]

Thanks, I am familiar with the lead paint issue, see here.

I am working on a method to recommend for re-coating these homes with something that will work.

Previously painters used a power washer to remove the loose and flaky paint as well as the bio organic growth (BOG), mold and mildew. EPA lead safe work practices (LSWP) require the water from power washing to be filtered and flushed. This has caused the compliant painters to actually collect the water and store it in barrels to be disposed of later by filtering the water and flushing the filtered water.

I am in favor of scraping and sanding the loose paint using LSWP and then using minimal scrubbing to remove the BOG.

Then we get to the re coating, you are recommending a stain? Solid body stain?

As NWA pointed out the weatherboards are old growth cypress, the reside option is either new growth cypress which does not have the durability of the old growth and typically fails in a couple of years or fiber cement. Neither of which is palatable to the pocket book, aesthetic or HDLC.

Not being a painter I am not familiar with solid body stains which would look good over existing siding with some paint intact.

BTW, we are doing a project next year using western red cedar, will post photos on that as it progresses.

I only wish to inform everyone that bare wood exposed to sunlight causes the surface of the wood, any wood, to erode. This damage occurs quickly, as in a week or two. The lignin breaks down from photochemical degradation, and as the lignin in the wood surface is destroyed it releases the wood fibers on the surface to depart. This same erosion can also be precipitaed by harsh chemicals that can destroy the lignin, and things like paint strippers, chlorine bleach, and degeasers all can cause that. Therefore in order to have any reasonable expectation that the new coats of solid stain or paint don't peel and crack, the wood surface of the siding must be primed quickly once the paint removal process has been completed. (solid stain is paint because both form paint films)

[Bill Robinson]

Bill,

The Architects I work with on Historic projects with the same type of issues always spec SW duration for this type of work. Usually preped by hand scraping and sanding the edges of the remaining (and well adhered) existing paint so it has a feathered edge. Work very well at filling in fine "alligator" type cracks too.

I went to a job we did about 5 years ago to look at some other work and the paint has held up very well. I believe we used a SW oil based primer for the bare wood.

Thanks Jason, I was wondering how the escaping lignin issue would impact old wood.

We are using Duration for top coat now and a SW oil based primer.

Just wondering what else to consider.

Thanks,

[CedarEd]

The color pigments in paints and stains, even penetrating oil stains, only adere to the top most surface fibers of any wood surface. The oil in the stain may penetrate, but the color pigments still never penetrate below the top most surface fibers. It is for this reason that sunlight exposure causes paint failures regardless of the paint itself. The primer or top coats pigments only adhere to those surface fibers, if the fibers are no longer held in place by the sun damaged lignin, they depart, and they take the pigments, the paint, along with them.

Paint has been able to protect wood like siding for hundreds of years, so long as it adheres to the wood, and doesn't peel. Paint is designed to slowly wear away, and so long as it is top coated/recoated before the erosion exposes the prime coat, the wood is protected. For a better explanation read the paper titled, " Don't get (SUN) burned " published in the Journal of Architectural Coatings - July 2005, and authored by R. Sam Williams,Ph.D. USDA Forest Service, Forest Products Laboratory. Sam was the senior scientist in the 'Wood Surface Chemistry' work unit for many years, and was responsible for the science that proved the correlation between pre-weathered wood, and paint failures.

Scuff sanding bare wood, is helpful in removing these partially detached wood surface fibers, if the wood surface has been exposed to the elements for a few weeks.

[No faux zone]

Ed, is there any scientific data out there saying that the damage from sun can go past the surface it's hitting? I guess what I'm wondering is, what is the difference if old growth wood is bare for a few weeks or a few years?

I would assume that there's very little difference once the damage has been done. I'm sure there might be some degree of difference with erosion of the damaged fibers from wind and rain allowing the sun light to further degraded newly exposed fibers.

From my experience, it is very minimal.

[dgbldr]

Bill,

Depending on the condition of the paint, you may wish to consider alkaline stripping. It's done fairly routinely on historical buildings. It may be cheaper than mechanical scraping/sanding, etc. in some instances. It's also very RRP-friendly when done correctly.

My observation from experience is just as CedarEd says. Any wood that has had significant sun exposure uncoated is basically toast. You need to sand it to fresh wood or replace. Using good primer and SW Duration is good practice, but of limited help if the wood is deteriorated.

[CedarEd]

Ed, is there any scientific data out there saying that the damage from sun can go past the surface it's hitting? I guess what I'm wondering is, what is the difference if old growth wood is bare for a few weeks or a few years?

I would assume that there's very little difference once the damage has been done. I'm sure there might be some degree of difference with erosion of the damaged fibers from wind and rain allowing the sun light to further degraded newly exposed fibers.

From my experience, it is very minimal.

Some wood species resist the photochemical degradation from sunlight slightly better than others, but because all wood is about 30-40% Lignin, it is all subjected to the same kind of problem, the surface erosion caused by weathering. The fibers on the surface are most effected because the sunlight destoys only the lignin it can reach, and the sunlight does not penetrate the solid wood. So whehter the tree is young or old, the damage is to the very surface of the wood that sunlight can reach. If the wood is badly split, splintered and checked, the sunlight reaches deeper into those areas, which is why simply sanding a severely weathered wood surface to expose new undamaged wood will not be as effective as it will on wood that has not been severely weathered. But make no mistake about the damage causd by sunlight, the tests have shown that even 1 week of sunlight will eventually cause some paint failure later on, it just may take 18 years to become visible. The 16 weeks of exposure done to Western Red Cedar siding before painting had the paint cracking and peeling in only 3 years. So the effect is determined to a degree by how much pre-weathering the wood is subjected to in all cases.

Also don't forget that washing wood surfaces with harsh chemicals and powerful cleaning agents does the same kind of damage. So rinse the surfaces well, use mild, not harsh cleaners, and dilute chemicals like bleach 4 or 5 parts water to 1 of bleach. They FPL used to say that 3 parts to 1 was sufficient for chlorine bleach, but they have learned that even that amount of diluting was unable to prevent the damage.

FYI - Lignin is used in the mud of oil well drilling, and it is what binds the pills you get from the pharmacist in the drug store together, so they dissolve readily after you swallow them.