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Dont understand undercoats


 
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LindsayO

from United Kingdom

Joined: 29 Dec 2006
Posts: 88
Location: Edinburgh,
United Kingdom

PostPosted: Mon Apr 16, 2007 7:21 pm Reply with quote

Hi,
I'm trying to get my head around undercoats and have lots of questions

I'm repainting the whole house, walls some painted over with primer, some painted with emulsion, ceilings previously painted with flat matt emulsion, woodwork, previously painted with gloss.

What is the difference between primer and undercoat? - I'm using Zinsser prime to cover lead paint, distemper and water-stains, do I need an undercaot as well as a primer?

Do all of these surfaces need undercoats? or can I gloss over previous glossed woodwork? I want to use water-based paints wherever possible, can I put water-based gloss over oil-based gloss?

For the walls and ceilings that I want to paint with a good quality emulsion can I use homebase magnolia emulsion as an undercoat or do I buy a special undercoat or use two coats of the topcoat?



Thanks guys, I'm clueless
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Zampa

from United Kingdom

Joined: 14 Mar 2005
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Location: United Kingdom
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PostPosted: Mon Apr 16, 2007 9:50 pm Reply with quote

What is the difference between primer and undercoat? - I'm using Zinsser prime to cover lead paint, distemper and water-stains, do I need an undercaot as well as a primer?

A primer is designed to a) soak into and adhere to a surface b) react chemically with it etching on to surfaces such as some shiny metals..

Undercoat, being very thick fill tiny surface irregularites, blots out the existing colour and provides a thicker film build, thus protecting the surface better..it also 'keys' better to sufaces such as previously painted ..like gloss because of its courser pigments and the different oils or resins that the pigment 'sits' in.

can I gloss over previous glossed woodwork?

Its not good practice..for the above reasons..gloss is made up of mainly oil..less pigment and doesnt have such good adhesion to shiny surfaces..

I want to use water-based paints wherever possible, can I put water-based gloss over oil-based gloss?

Acording to some books...yes..however, you should rub the surface down really well to provide a key..they dont cover very well, ...so if you need to coats you may as well make one of them an undercoat..(water based)

For the walls and ceilings that I want to paint with a good quality emulsion can I use homebase magnolia emulsion as an undercoat or do I buy a special undercoat or use two coats of the topcoat?

Dont bother with the home brand stuff..and what you put on them will depend on what i on them now..

Which is?
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LindsayO

from United Kingdom

Joined: 29 Dec 2006
Posts: 88
Location: Edinburgh,
United Kingdom

PostPosted: Mon Apr 16, 2007 10:22 pm Reply with quote

thank you Zampa, very helpful.

If I understand you correctly I should always use an undercoat then.
I'm still confused about what undercoat to use with gloss, and I'm not sure if my plan to stick to water-based paint is a good one, if I can't use water-based gloss over oil-based gloss. So if I go with oil-based gloss over old gloss, should I still use an undercoat?

Also you asked what I have on the walls that I want to put emulsion on. It is quite a selection

Kitchen: Zinsset Bullseye over old lead paint
Living room: Zinsser Bullseye over distemper
Hallway: one wall, Zinsser Bullseye over scrubbed plaster (scrubbed off the distemper), other wall, previously painted wallpaper, painted with homebase magnolia
Stairway: plaster painted with homebase magnolia, that wasnt properly prepared, needs scrapes filling in and smoothing over
Bedrooms, lining paper painted with blood red emulsion, lining paper painted with pale cheap emulsion
Master bedroom, has wallpaper that I dont like that is nicely stuck on, I'm tempted to try and paint over it
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spice

from United Kingdom

Joined: 11 Mar 2006
Posts: 569
Location: London,
United Kingdom

PostPosted: Tue Apr 17, 2007 6:25 am Reply with quote

Some good information there from Zampa. icon_biggrin.gif

Hopefully they should be starting to improve on the waterbased Undercoat/gloss etc, as IMO they are kr@p, you have to do a few coats to get a decent finish.
Its advisable not to gloss over existing gloss, as it will flake and come off.
I always use oilbase when I can, and its better to use oilbase on new/bare wood, further down the line you could use waterbase.

If your woodwork is quite grotty, I would use the undercoat on its own, not the two in one undercoat/gloss, infact me personally even if its not grotty lol I would still U/c, then finish with your gloss etc.

if you went for an eggshell or satin finish, you could get away with not undercoating, but again depending on your woodwork and how nice a finish you want.

As for your emulsion paint, its is a great idea to cover strong colour with a coat of white emulsion, but it might be better if you stick with the one brand name.
Dont foget you get what you pay for, and if you buy a brand like dulux, a 5 ltr tin, the time you thin it down etc, you get more from the can.
I know people sometimes thinks it works out cheaper to buy a cheapo paint for an undercoat on the walls as such, but sometimes if you mix 2 different brands, you can get a reaction.

There are so many offers at the moment with Dulux, I am doing a HSL at the moment, and there existing colour on the HSL is dark blue
icon_eek.gif , and I am using Dulux like I normally do, but I am still putting a coat of dulux white emulsion over the blue first, as I know there will be no combacks. icon_biggrin.gif
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LindsayO

from United Kingdom

Joined: 29 Dec 2006
Posts: 88
Location: Edinburgh,
United Kingdom

PostPosted: Tue Apr 17, 2007 9:07 am Reply with quote

Thank you so much Spike, I just want to see if I understand correctly.
It sounds like the best plan is to
1. Stick with the same brand on any surface - I'm happy to do that and toss the HB paint altogether
2. Always use an undercoat on every surface
3. Use oil-based gloss in preference to water-based gloss

I still have questions though

Do I need to buy a paint that calls itself Undercoat, it sounds like for walls that I emulsion, I can use white emulsion as an undercoat, but for woodwork that I gloss I need to buy a paint other than gloss that is sold as undercoat

Also I understand that dulux is regarded as a good quality paint, what about Johnstone's?
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Third_Eye

from United Kingdom

Joined: 11 Apr 2006
Posts: 1080
Location: United Kingdom
Thanked: 4 times

PostPosted: Tue Apr 17, 2007 11:46 am Reply with quote

Way i see it all "LindsayO" is Emulsions DO NOT need any Undercoats (unless in unique circumstances). Emulsions are mainly for Internal walls & cielings.

Emulsions can be applied over each other without an Undercoat. (though beware of matt over silk crocidile effect).

Primers (can be used on masonry), Undercoats, Gloss, Satinwood & Eggshell are all mainly for wood or metal or plastics, if the paints mention they allow it to be. (though in unique circumstances all paints just mentioned can be used on walls & cielings internally.)

Masonry Paints are specifically for roughcast, pebbledash & render etc.

There are Primers, Undercoats, Satin, Gloss are specifically made for only Internal work. Some are made for only external work. Some are made for both.

In other words the GENERAL rule is Emulsions for walls & cielings. Primers, Undercoats, Satin, Gloss are for woodwork, metal or plastic. Masonry Paints are for external walls. Though YOU MUST READ every tin of paint before using in terms of what it can & cannot do. Sometimes you even have to get even more litireture of the net before using your desired paint as sometimes there is further reading, which is not on the tins. This can cause major mistakes to be made.
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Zampa

from United Kingdom

Joined: 14 Mar 2005
Posts: 5779
Location: United Kingdom
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PostPosted: Tue Apr 17, 2007 5:45 pm Reply with quote

Quote:
Thank you so much Spike,
icon_lol.gif
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spice

from United Kingdom

Joined: 11 Mar 2006
Posts: 569
Location: London,
United Kingdom

PostPosted: Tue Apr 17, 2007 6:03 pm Reply with quote

Zampa wrote:
Quote:
Thank you so much Spike,
icon_lol.gif



Ah, Bless, its an easy mistake to make. icon_wink.gif
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Zampa

from United Kingdom

Joined: 14 Mar 2005
Posts: 5779
Location: United Kingdom
Thanked: 25 times

PostPosted: Tue Apr 17, 2007 6:17 pm Reply with quote

Thing is..I now have an image of you looking like spike milligan.. icon_lol.gif icon_wink.gif
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LindsayO

from United Kingdom

Joined: 29 Dec 2006
Posts: 88
Location: Edinburgh,
United Kingdom

PostPosted: Tue Apr 17, 2007 6:36 pm Reply with quote

Oops sorry spice
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