Decking paint failure, why and what to do about it?

[ProDave]

Last year we made a decision we wanted all the exterior wood on the house the same colour.  That is the timber wall cladding on the sun room, various areas of timber decking and all the handrails etc.  To buy off the shelf paint for walls and decking limits you to a very few colours usually a variation on brown or orange, which we did not want.  So we opted to use Valspar, the one that B&Q sell where they mix the colour while you wait from a vast pallet and they do a whole lot of different types of paint.

 

So we bought some "garden exterior paint" for the walls of the sun room and "decking paint" for the decking.  the whole lot was painted last year as we did it.

 

The garden exterior paint on the sun room cladding is performing well, just over 1 year and 1 winter it is as good as the day it was applied.

 

Not so the decking paint.  Even before the first winter was finished, much of the decking paint has flaked off.  Total disaster.  So a solution or alternative has to be found before we continue.

 

Now the decking wood was all new, the paint was applied to fresh clean dry decking boards in good warm dry summer weather.  Some has done better than others, e.g. the ramp to the front door is the worst, while the deck at the side of the house leading to the back door is a lot better but some areas failing.

 

What is the cause of this?  Rubbish paint?  Or wrong application?

 

At the time, timber has hard to get, so we have deck planks from 3 different sources, they are not all the same.

 

So could it be the treatment the deck boards came with?  They are not just bare plain timber (like the wall cladding boards on the sun room)  So could it be this manufacturers treatment is why the deck paint has failed to adhere?

 

I will post pictures later.

 

As a test, to try and find a solution, I have stripped the failed paint from one small section, a 2 plank wide step, and re painted not with decking paint, but the garden exterior paint.  This is a test to see if it lasts better or fails in the same way.

 

Any advice please?

[Conor]

We spent a fortune on decking paint (something like £60 for a tin, needed three or four of them) and it was scuffed in a few days, chipping off in a week and fully peeling off on the damper spots within the year.

 

So the next year I lifted all the boards, flipped them to the unpainted side and just treated with clear decking stain. 

 

Forget about paint for a deck.

Edited September 26, 2022 by Conor

[Modernista]

I'm no chemist but some preservative treated wood contains water repellent additives (particularly if it's expected to be the final finish). I've often wondered how that works with water based or water soluble wood paints (and stains). Perhaps one or more of the decking batches had been so treated and the water repellency didn't allow the paint to function as intended? Especially as it sounds patchy / possibly at high wear areas?

[markc]

Depends on what the decking timber is treated with and how long it has had to dry out after treatment. You would not try to paint green timber and pressure treatment pushes loads of moisture back in.

[Radian]

As a general rule I never use the type of garden paints that form a skin. The only sort that seems to stay put is the type of paint intended for rough sawn timber i.e. fence paint. These just fade away rather than peel. That means they need no great amount of prep before repainting - which is always inevitable. Just a quick brush off and slap on some more. Much cheaper too, and often has a certain amount of wax content that shucks water off for a while.

[SteamyTea]

1 hour ago, Modernista said:

I'm no chemist but some preservative treated wood contains water repellent additives (particularly if it's expected to be the final finish). I've often wondered how that works with water based or water soluble wood paints (and stains). Perhaps one or more of the decking batches had been so treated and the water repellency didn't allow the paint to function as intended? Especially as it sounds patchy / possibly at high wear areas?

Water repellency works by having the opposite polarity to water at one end of the polymer. The other end is attracted to water, or carbon.

When water based paints dry, the water is, in effect, pumped from the hydrophobic end to the hydrophilic end, it is the hydrophilic end that is attached to the substrate i.e. the wood.

This causes an excess of water molecules to form and evaporate away. As the water evaporates from the polymer, the polymer shrinks and closes the pores and gaps that once held the water molecules. This is why water based paint needs to be very well stirred. The ratio of polymer to water molecules is important (so not always a good idea to water it down).

Once fully dried, those pores are now to small to allow a liquid water molecule into them, and because of the opposite electrical charge, the water is repelled.

So knowing that, the main reason that these types of paints often fail is not to do with the paint at all, more to do with the substrate they are trying to attach to.

If the substrate is damp, too smooth, or very cold, there will be a surplus of liquid water at that end, this will hamper evaporation, so the paint will not dry properly. High air humidity has a similar effect.

It may initially look and feel fine, but under the skin, it is still liquid, but because the skin is hydrophobic, the water molecules cannot escape, so just collect between the end of the polymer chain and the substrate.

Come the next bit of hot/cold weather cycling, the outer skin expands and contracts, eventually a time will come when the movement passes the yield point of the polymer and a tiny crack forms. 

Now the fun starts.

The free, unwanted water molecules are now able to evaporate, leaving a hydrophobic surface to repel more water, but more importantly, not stick to the substrate, that wants the hydrophilic end of the molecule.

Water molecules now shift towards this hydrophilic end and get trapped, then more expansion and contraction causes more cracks, fissures and holes at an increasing rate.

The failure accelerates and soon there are large flakes falling off.

 

So what to do.

Basically follow the manufactures instructions. 

They employ clever people that know how their paint works, how to apply it, and when to apply it.

Do as they say.

Paint onto slightly roughened surface (larger area to stick to), make sure it is not too hot  or cold, not in direct sunlight, the correct moisture and the air is within the 40 to 70%RH.

So paint under cover.

 

Oil and PU based paints are different, the above is for water based.

Oil paints need an etching primer, then they chemically bond to that, and are generally more elastic, so can cope with greater expansion and contraction.

PU paints are a bit more weird. They need moisture to cure, the hydrogen in water combined with the polymer molecules, the oxygen causes a harder skin though oxidisation. But again, too much water during application and they fail.

 

I wish I had stayed awake during chemistry lessons, but I found it very boring.

I still do. 20 oddly made up words, just rearranged in an odd fashioned to part of an elements name.

It is more Scrabble than science.

It is for people with good memories and no imagination.

[ProDave]

So reading that post above, the point I take from it, is the paint has stuck nicely to my timber cladding because it was rough sawn timber.  It has not stuck to my decking because it was very smooth.

 

Is that close to the mark?

 

The bit I have re done as an experiment, I used a wire brush to clean the remains off, about the only thing I could find that would get into the grooves.  That will have roughened it up a bit.

 

So what should one do with freshly bought deck planks that you want to paint, to ensure the paint sticks?

[SteamyTea]

5 minutes ago, ProDave said:

So reading that post above, the point I take from it, is the paint has stuck nicely to my timber cladding because it was rough sawn timber.  It has not stuck to my decking because it was very smooth.

 

Is that close to the mark

Probably.

Was your decking timber treated?

Some treatments try to create a polar repellent on the air side, then when you paint them, the paint dries the wrong way around, so acts as a sponge, then it will never last.

Other treatments just try to kill fungi and bacteria, not sure what their polarisation is, but it will have one, and Sod's Law say it is the wrong way around.

Other treatments may just form a barrier, think wax. These only mechanically stick as they are, in effect, just a layer that repels water, all over.

 

Hydrogen is positively charged and oxygen is negatively charged, but they make a covalent bond, so a slight positive charge on the surface, but no overall charge internally, why it is a hard molecule to crack apart and has fantastic properties.

It also dissolved almost everything it touches.

[ProDave]

What treatment was on them, is not something I asked, nobody seems to, "Can I have some decking boards please"?

 

 

[Iceverge]

One trick I found with airtightness paint which when going in full strength first coat was that it was reluctant to stick to the substrate and peeled off in big wobbly strips. 

 

Cutting it with water was very effective in making a primer and it stuck well there after. 

 

Maybe make a very thin solution of your paint as a primer using a suitable solvent/water depending on its base. 

[recoveringbuilder]

We used ronseal decking paint last year and like @ProDaveit has peeled off BUT only on the part that we use regularly the rest of it is fine. It is now all slippery when it’s been raining something I always said I would hate. I have a friend who only ever oils her decking and it’s never slippery but unfortunately I wanted it grey, should have gone with the composite instead of trying to penny pinch

[ProDave]

4 minutes ago, recoveringbuilder said:

should have gone with the composite instead of trying to penny pinch

This is what I am trying to work out.  We have the balcony and an area of decking to do below that.  I don't want to jump in now and buy more until I know what I am going to buy and paint it with is going to last.  That might mean the balcony does not get a "floor" until next year.

 

I might ask the merchants if I can buy completely untreated decking to be sure that is not the issue with painting it.

[saveasteading]

I have noticed a big change in timber preserver treatment. Traditionally, tantalised timber was quite oily, and it penetrated several mm into the pores due to vacuum treatment, even on side faces. Then it became less toxic, and more water based, but still penetrated. Recently (based on what we have received) it seems only to have a surface treatment.

Perhaps you have a variety of these, including the original which is least accommodating to paint.