Old Chestnut: Best Way to Apply Water Based Paint Over Oil Based?

Discussion in 'Decorating and Painting' started by morningster, 14 Oct 2018.

  1. morningster

    morningster

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    Yes, it's a well-discussed topic but after reading forums, articles and watching vids all over the place, I still can't find a satisfactory answer to this...

    Front door was painted with oil paint long ago, now there's almost no sheen left on it, it's quite chipped, a little cracked... I want to paint exterior aspect with quick-drying water-based satin or low-sheen paint but I want to make sure it sticks well. Also, btw, door is white and I want to apply a light-to-mid green paint over it.

    This is some of the advice I've found on this topic:

    (a) I should clean the old surface and give it a good key - 120 grit and up;
    (b) I should apply primer and/or undercoat;
    (c) The undercoat can be water-based or oil-based.. but preferrably oil-based;
    (d) Then paint as usual with something like Weathershield Quick Dry Satin.

    I don't have lots of readies to spend, so if I can make use of my tins of Zinsser BIN and/or Leyland Trade Acrylic Primer Undercoat, that'd be great. I could plump for some Zinsser 123 or Sikkens Rubbol BL, as I've heard good things about them?

    I'd still need to buy a topcoat, obviously.

    So what's the best way to go about this, how many coats for each layer do I need? Do I really need primer? Does painting a light-to-mid colour over white involve any extra faff? Is the answer to "what's the meaning of life" really 42?

    Advice on exterior paint brands would be helpful too.
     
    Last edited: 14 Oct 2018
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  3. morningster

    morningster

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    Bump. Not a popular topic? :unsure:
     
  4. wwwebber

    wwwebber

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    Just painting some outside window frames. Old paint flaking off etc. I simply bought some 10 year guaranteed external paint that claimed it didn't need an undercoat / primer - just two coats and you're done according to the tin.

    I sanded the frame of course beforehand. Bare wood would need more coats too.
     
  5. morningster

    morningster

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    Is that Dulux Weathershield or similar? Is it water-based or oil?
     
    Last edited: 16 Oct 2018
  6. wwwebber

    wwwebber

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  7. opps

    opps

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    Can you describe the where the cracks are? Is the door a mortice and tennon door. Are the cracks where the rails meet the styles?


    a) If the door is fairly clean, a sand with 150 or 180 grit paper should suffice. Consider using 2 pack filler to fill the chips.

    b) There is no need to apply primer unless you have patches of bare wood.

    c) I personally prefer oil based undercoats. I find they flow better and are easier to sand flat. Most waterbased undercoats do not like being sanded. You mention that you already have some Leyland Trade acrylic undercoat, it is one of the only WB undercoats that sands nicely, if you use 180 grit silicone carbide paper. The Leyland will be fine. BIN is great but you will not be able to brush it to a decent standard.

    d) If you want a satin finish then Weathershield will be fine.


    A dark green would require a grey under coat first. You may get away with a couple of coats of top coat if the green isn't too dark.


    The answer is indeed 42, but we clearly do not understand the question...
     
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  9. morningster

    morningster

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    Thanks, opps.

    Yes, where rail and stile meet. Otherwise, some are along inner edges of the four recessed panels, but most problem areas are chips, big and small.

    Yes, that's why I was thinking of using the BIN, because it's solvent-based - sticks well to all kinds of surface and gives a tough shellacked finish.. but it's definitely not fun to work with as it dries so quickly.

    If I can get away with using Leyland Trade Acrylic U/C over the old oil paint, I'm happy with that.

    Colour-wise, this is the kind of green I'm looking to use, so not dark at all.

    So, 2 coats of U/C and 2 of top?
     
  10. opps

    opps

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    Rail and style cracks can be a mare to eliminate if you want to maintain a flat finish.

    The recessed panels can be caulked, a better option however would be to use a MS polymer such as CT1 (about a tenner per tube though). Just so that you are aware, never apply oil based paints straight over MS polymers without applying a coat of WB U/C first. OB directly over them results in the paint staying tacky for weeks. They are however great products, they are as flexible as silicone sealants but with the added advantage of being paintable.


    In theory BIN should only be used as a spot primer on exterior woodwork. Not sure why though. Perhaps it is something to do with UV light dissipation?

    Other than spot priming, I would only apply one coat of the leyland and then sand it as flat as possible using 180 grit silicon carbide paper (the grey stuff). It doesn't matter if you sand through it slightly in a couple of places.

    Given the shade of green. Hopefully 2 coats of satinwood will suffice. Theoretically you don't even need to apply the leyland U/C, satinwood and eggshell are "self undercoating". I just find the Leyland to be a useful "search coat". As one of the few acrylic paints that is easy to sand, I'd rather pick up on any imperfections (eg high spots, cracks) before applying the (less easy to sand) top coats.

    I would also recommend that you don't paint the door too late in the day. The evening dew can make the sheen level become more matt. I seldom use waterbased paints outdoors but oil based gloss will look like satin wood if you apply it a 4pm (at this time of the year). TBH I am not sure how late in the day you can apply WB exterior paints but given that they work via evaporation and coalescence you may even find that the paint fails as a result of the moisture deposited by the dew.

    Best of luck
     
  11. morningster

    morningster

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    Forgot to say thanks for all the useful - and scientifically-precise (!) - info, opps.

    Still not got round to doing more than preparing and filling the crack and chips - one chip takes up most of the bottom rail... I use either Everbuild ready-mixed wood filler - found it surprisingly good for a non high-performance filler - for lesser blemishes and Metolux Styrene-Free 2-Part Wood Filler (Polyester-based) for rail-stile cracks and larger bits. It does go off real quick, though!

    Now that I've procrastinated (my middle name, don't you know) and it's end October, I'm not sure if I can still paint the door with the low temps and humidity. I rather hoped for more of an Indian summer than the few, disparate days of warmth we got down here in the SE earlier in the month. I'm that daft.

    Do you get much exterior painting work this time of year?
     
  12. opps

    opps

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    What is the Metolux like to sand? I am London based and I suspect that the Leyland SDM's (a decorators' merchant not related to leyland the paint brand) own brand 2 pack filler is made by Metolux. I base my assumption on the fact that the Leyland SDM branded epoxy wood hardener is made by TimbaBuild- part of the same group as Metolux. The Leyland SDM 2 pack filler clogs the paper on my sanders!

    I am currently working out doors trying to repaint/repair a client's conservatory in west London. I did suggest that he rip it down and have a new one erected but he likes it and is willing to pay silly money to have it painted to a high finish.

    WP_20181022_09_47_56_Pro.jpg

    I don't like working outdoors at this time of the year. Largely because I don't like the cold... In all seriousness, the TimbaBuild epoxy products that I am using to splice in the new timber take ages to harden. The Dulux Trade Weathershield (waterbased) primer is still tacky the next day, however the oil based undercoats and gloss are fine (especially if you add terebene).

    The main issue with working outdoors this late in the year is that it is difficult to dry wet timber. You can aim your heat gun at a wet bit of wood but even if you hold the gun static for five minutes, once you take the gun away, the moisture in timber rises back to the surface.

    A couple of years ago, I had to repaint an exterior from late November to February. Some days your hands stuck to the scaffold poles because they were so cold. The job took 35-50% longer than it would have had it been summer but it seems to have lasted thus far.

    Apologies for going off topic, apply your waterbased paints but perhaps leave a heated fan trained on it to help it "dry".
     
  13. morningster

    morningster

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    Oh my, you do have your work cut out for you with that project. My sympathies. Still, if the wood is wet due to rot, isn't there an abundance of products from the likes of Ronseal and Everbuild to deal with that? Of course, if it's due to rain and ambient humidity, that's summat else...

    No idea re sanding Metolux as I haven't sanded it yet! ;) Applied it after fully mixing what seemed to me a proper ratio of brown stuff to white stuff but even after more than adequate time had passed, some fills seemed almost tacky while others seemed ok; I think the "tacky" ones were filled when the stuff was really going off and working it with the filling/deco knife and plastic thingamy was a labour of tearing rather than spreading. Seems fine now. We used Tetrion Woodfil before, but wreaks to high Heaven of styrene which I was trying to avoid; this "styrene-free" filler smells pretty much the same, feels like it's going up your sinuses and brain-wards to do interesting things.

    I do patronize my local Leyland SDM and I was indeed told by Chemfix (who make these styrene-free fillers) that Leyland sell their stuff under their own brand name but when I looked around, I could only find styrene-based products; now I realise they must have been referring to their epoxy products, not polyester. Hmmm...

    Honestly, I wish I didn't have to shop and cook and the like cos I'd get more done. Plus someone in the household had the bright idea to buy a kitten; he's somewhat taken over the house and opening doors is a dodgy proposition as he's bound to bolt. Excuses, of course...

    Will try to get this job done before the cold bites too hard.

    All the best with your own outdoor project!
     
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