Brush strokes visible in linseed oil paint on metal

[malcolm]

My linseed oil paint finally arrived and I've been painting my metal crittall windows which have been primed with cold galvanising paint. The finish is rough. I applied the paint thinly and despite taking the whole day to dry to tacky the paint didn't flow at all. The photo below doesn't really show the issue which is clearly visible brush marks rather than the corrosion marks.

Is there a trick to avoiding brush marks? I'll give it a good sand in a few days and buy a really soft brush and try again, but I wonder if I applied the paint too thinly. I applied the paint as I would modern gloss paint (the one before the new water based and non-drip) which flows very well.

I didn't have the trouble on wood which soaks the linseed paint and makes it go flat, only metal.

 

[88v8]

Cold galvanising, is that some sort of zinc-rich?

Errrm, why would you want to use linseed on metal?
It strikes me as a bit self-flagellating.

If I were feeling tight-fisted I'd be using epoxy chassis paint, or ideally that epoxy Two-pack I've posted before, or at a pinch Smoothrite although I'm not sure that's as durable as once was.
The two-pack, I've discovered, is not uv-stable, so in a year it takes on a lovely dusky texture, like the lead paint of olde.

Anyway, brush marks, will hold the dirt, make it look 'aged' sooner. Instant patina.
Good.

Ivor

 

[mims&pip]

I have to agree with Ivor. The Crittall website says this about renovating older windows -

"Zinc-rich paint should then be applied to any part with damage to galvanised finish. The frame can then be painted with zinc phosphate-rich primer and undercoat and external quality paint of the type developed for metal."

In addition to a number of Victorian timber sashes, we have numerous Crittall windows and doors here (iirc 13 windows including two large square bays, 4 doors plus a pair of French doors), dating I believe from when the house was extended in 1924. The previous owners had these repainted around twelve years ago and (according to a neighbour, a painter/decorator that carried out the work) ordinary external eggshell for wood/metal was used. Close inspection reveals very minimal (if any tbh) signs of external deterioration and a fairly smooth finish with few visible brush strokes......shame as the colour is blooming awful :roll: :lol:

Mims x

 

[malcolm]

The primer is Bilt Hamber Electrox which is 95% zinc by weight and is unusually heavy. I used it on my latest car restoration and it does appear to protect very well. A part underneath was accidentally scraped back to bare metal about 5 years ago and should still be clean by the time I get around to doing something about it. The primer requires a shot blasted metal finish as it doesn't stick well.

I plastered a lath ceiling this morning (and screeded a floor at the same time) so self-flagellating is a possible excuse. Mostly I wanted the paint on the frame to be serviceable in many years rather than crack, and I wanted it to go a bit matt. I guess it was originally painted in linseed oil paint in 1910.

The finish seems to be improving after another day of drying. A light wet sand in a few days and another coat should hopefully do the job. One of the paint suppliers suggests applying the paint with a cloth for a thinner brush mark free finish (then presumably drop the cloth in a bucket of water so it doesn't catch fire).

 

[Lime]

I agree with the comments about using linseed oil paint on the primer.

Linseed oil paint should be as thin as you can get it and well brushed on, back and forth several times, so much so brush marks are too thin to be seen.

 

[Toby Newell]

I write as a man with several hundred pounds worth of brushes, some of whom I consider amongst my best friends and who has coated in his short lifetime several square miles of flat area and several tens of linear miles (possibly much more) of isolated surfaces.

I applied the paint thinly and despite taking the whole day to dry to tacky the paint didn't flow at all

The artefact of 'brush marks' is caused when the finish does not have enough time to level out and flow before it starts to set. That's it. No other reason. There are however several factors that cause this.

1. Viscosity of medium too thick. (medium too thick to move before it starts to cure fixing it in place)
Solution: Thin medium with appropriate solvent until correct flow characteristics are achieved and / or increase ambient temperature, temperature of the medium or substrate or a combination of all of these.

2. Not enough material applied to surface. (medium may flow but not enough material to reach the next 'ridge' or line)
Solution: Apply more finish.

3. Incorrect application technique or tools. (using incorrect tool - brush- or incorrect technique, brushing either too little or too much)
Solution: Use an appropriate brush for the finish. Use correct application technique. Ensure enough finish is applied in a quick motion then feather the finish by lightly stroking the surface in as few movements as possible, some very thick traditional finishes will require more vigorous brushing to distribute the finish, experience tells when one should stop when they feel the 'pull' reach a certain strength, there is no way to explain this other than by direct learning. Over brushing will make the solvents evaporate faster and will lead to no.1, the local viscosity to be too thick meaning the finish will never level correctly. Too little brushing may lead to too thick ridges which may not have time to flow before they start to set or their viscosity reduces to a point where they cannot move easily, feathering lightly reduces the height of these ridges and creates smaller newer ridges in between. Remember to keep your brush clean as is can gum up whilst working, regular washing out is sometime required during the job.

4. Sub optimal site conditions. (too hot, too cold, too humid, too dry, too much wind or a combination of all three, heat, humidity, ventilation)
Solution: Avoid working in windy conditions which can evaporate solvent quicker and reduce viscosity and hence material flow. Avoid working in too hot conditions which also evaporates the solvent quicker with the same result. Avoid working in too cold conditions, this normally has to do with final finish curing but peculiarly in the case of some old oil based finishes that are very viscous (and whose viscosity reduces dramatically as they are heated) working in the cold can make the finish very viscous and thus retard the flow rate, the finish does not have time to flow before the solvent evaporates. In this case you need to either gently warm the finish, the substrate being finished or heat the local area / wait for warmer ambient conditions.

5. Incompatible undercoat. (two materials may not be compatible and thus a 'reaction' can occur which can effect the drying, curing and thus flowing of the top coat)
Solution: Undertake tests using different undercoats to test compatibility.

If you have ascertained that both finishes (undercoat and topcoat) are compatible then the next step is to ensure you are applying enough finish to the surface. To ensure optimal flow and spread one always wants apply the thickest coat possible that will not pool, run or drip. If this doesn't work then you need to look at the viscosity of the material which in most cases involves adding more solvent but in the case of old oil finishes whose viscosity is also heat dependant also involves ambient temperature and temperature of the substrate (metal can get very hot or cold) temperature should be adjusted before solvent is added as too much solvent can adversely effect the drying and curing process and ultimately the dried film characteristics (durability for example)

Use a good quality, clean bush and remember to clean this out regularly as it gets 'gummed up' with partially dried finish, remember to shake and dry all the solvent off after cleaning otherwise your first few brush 'fills' will thin the paint as you brush.

Unless someone is there watching you and under what conditions you are working it is impossible to say what (if anything) you are doing wrong.

From experience (and looking at your photos which show the finish to be slightly 'rucked' and 'orange peely' I would firstly discount any incompatibility and adverse reaction by trying the exact same method on a different undercoat, only once you have proven these finishes work together can you move on to isolate your flow problems (if the finishes are incompatible it doesn't matter how perfectly you coat, the result will always be sub par) If the finishes do match then I might suspect you are applying slightly too little material and / or over brushing too much or that the site conditions are in direct sunlight or too windy, sometimes it is a subtle combination of many exacerbating factors. You want to put enough material on (you can remove excess as you work if it drips) and even it up as quickly as possible with as few strokes as possible, practice often makes perfect as they say, or at least makes an improvement.

I know this may seem overly complex but there are so many variables involved in finishing sometimes it is as much an art as it is a science. The trick is to try and be as scientific as possible by isolating one variable at a time so they can be eliminated and the problem discovered by inductive reasoning (never assume anything or what anyone has told you, often including the manufacturer and especially 'the tin' ! very often the product does not do what it says on the tin and the manufacturers know this) once you have worked out what was wrong next time you will instinctively have a better 'feel' for what the problem may be which will help you resolve that problem more quickly in the future.

It's just patience and common sense really, I hope you found some of that useful.

Toby

 

[malcolm]

Thanks Toby, and good timing. I wet flatted the window frames this morning and am aiming to try another coat this afternoon. Some of the problem was brush marks in the primer coat which dried too quickly, but I didn't notice the brush marks in the matt finish.

I'm nervous about applying the linseed paint too thickly (perhaps this is the problem) so will try thinning the next coat with a little turpentine and be a bit more careful with brushing.

 

[Toby Newell]

Yes. Always flat the primer coat with fine wet and dry being careful not to 'burn' through the edges.

Try keeping the paint inside for a day or two (if it is normal kept in a shed or other colder place) the (increase, sorry strike that) decrease in viscosity can be just enough to solve the problem. If you do add solvent add a little at a time, too much and you will have the opposite problem!

Best of luck.

T

 

[Toby Newell]

Final thought.

The more I look at the picture the more I see incorrectly cured finish, check the tack time, is it too long? Extended drying times often indicate incompatibility problems.

The golden rule is: Oil over cellulose = OK. Cellulose over oil = VERY NOT OK. But. Things aren't as simple as that.

How long did you leave the primer coat to dry? If it is xylene/cellulose based any residual solvent (xylene) would attack the linseed oil finish.

Are you using the linseed oil paint correctly? Is it designed to be used over metal? Or just wood? Remember, if the oil cannot penetrate the substrate (as wood is far more porous than metal which normally has very low porosity) then it may remain in the finish on the surface and retard correct curing.

Yes there are oil based paints which DO dry over metal but these have advanced siccatives added, maybe your 'traditional' linseed oil paint is a much simpler formulation and therefore is simply not compatible with metal coating.

Simple test. If there are NO incompatibility problems then wet and dry facing down of the problem area and overcoating with a more generous coat should sort it out. If there IS an incompatibility problem then facing down and overcoating normally will NOT cure the problem, the reaction will either be less or (occasionally) more but no amount of faffing about whatever you do will give a good finish.

Being uncommonly magnanimous I have not mentioned, as others have, the elephant in the room.

Why are you using a very highly engineered and sophisticated cold galvanisation metal paint underneath an ancient and very poor mechanical propertied linseed oil paint which possibly was not designed to be used as a metal finish?

Just thoughts.

 

[malcolm]

I did some experimenting this afternoon. Thinning the paint by 20% helped a lot. I still couldn't persuade it to flow, but a new soft brush helped. I ended up applying the paint very thinly. The brush marks are still there but much finer and they look like something a craftsman might have done.

The rucking in the first photo is mostly rust pitting. I don't think there are any paint reactions. I could have used a modern 2 pack paint but want something that will fade but not crack. The wood part will also be finished in linseed paint.

 

[Feltwell]

You're right to be nervous of applying linseed paint too thickly. It's very easy to do so, especially in any areas like the detail on wooden mouldings, and you end up with paint that is dry on top but still liquid underneath. When it eventually dries - which takes a very long time - it'll have more wrinkles than Nora Batty's stockings.

I'm still using linseed paint, but I do have something of a love/hate relationship with it. The softwood gates I did in a pale colour with Holkham's paint look awful only around 3 years on. I'm now using Linseed Paint Company's paint, harder to apply correctly but I'm getting better results and so far the finish seems to stand up to weathering better.

I admire your dedication to even think of using it on metal!