OK to leave hammerite undercoat

Hi, I fixed up some pending structural rust on my bike with hammerite red undercoat but don't really feel the need to colour match the bit I did (underneath). Can I just leave the undercoat exposed ? will this do the job ? I'm always seeing cars driving about will only undercoat.
Cheers,
Bruce.
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Hammerite undercoat, I don't know, but I thought that normal undercoats absorbed water
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geoff

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writes

Primers stick to surfaces but don't cover and have no surface themselves, either flat or protective Undercoats stick to primers, cover, and have surfaces that are flat but not protective Top coats stick to undercoats, don't cover, are only as flat as the surface underneath but have a protective surface.
Combination paints (eg primer/undercoats, "one coat" for decorating) muddy the water, but the above is close enough!
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Bob Mannix
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Bob Mannix wrote:

Most undercoats are the same as topcoats except they have a higher solids to resin ratio
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writes

Errr, so they are not the same! Higher solids, lower resin means better cover and flatter/matter nad no protective surface. More resin less solids means worse cover but a hard protective coat which is, I believe, what I said. There is a sense in which all paints are "the same with different ingredients" of course!
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Bob Mannix wrote:

*less* protective surface rather than none.
. More resin less solids

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