I hate to sound discouraging, but it is generally considered (and my experiences bear this out!) that you cannot expect paint to stay in place on concrete unless the concrete is 100% dry.
This does not address your question about the surface of the paint becoming wet, but other contributors have explained the probable cause of this problem.
Ordinary concrete (as opposed to paving mix, or commercially made/delivered air-entrained concrete, is porous. So ground water is drawn up through it by capillary action.
What I go on to say assumes that you used a general-purpose concrete mix for your floor slab and did not lay it, including the sides of the slab, on a damp-proof membrane.
In practice, even inside a building such as a garage, rising damp may not be a nuisance provided the surface of the concrete is exposed to air currents, which allow the water rising to the surface to evaporate as fast as it appears. But if the surface is covered with an impermeable material (vinyl floor covering, concrete floor paint etc) the water cannot evaporate, and the capillary pressure either causes mould to grow under the floor covering (which may lift slightly in places) or simply lifts a lighter covering such as paint right off the surface. It tends to lift in patches, which then relieve the capillary pressure, so, depending on how much capillary action there is, this may leave substantial painted areas unaffected..
I imagine that this also applies to the lacquer commonly used to beautify and "seal" the surface of pattern-imprinted-concrete driveways, as well as to garage floor paint. It would not be realistic to expect a concrete driveway to be cast in expensive air-entrained concrete (I don't know about the concrete used for pattern-imprinting, but my neighbour's drive of this type sheds its lacquer continuously and needs re-treating at least every three years). Nor could a driveway slab be laid on a DPM without making the job much more complex/expensive.
12 years ago we cast a floor slab for a sizeable wooden outbuilding. This stands on the edges of the slab, and is strapped down to avoid any risk of gales etc dislodging it. We laid DPC strips for it to stand on, but this may have been unnecessary because the strong concrete mix that we used has proved to resist rising damp. The building is not specially ventilated, yet the floor is always very dry. Unpainted steel tools and equipment have not shown any surface rusting. The concrete was laid on a sub-base as outlined below. The ground below the sub-base is clay and we are at only 75 metres above sea level, so the clay is rarely dry and the ground water level is usually high. The building itself is of a much better and sturdier construction than an ordinary garden shed, and very draught-proof. The boarding is t&g so, apart from around the door, there are no draught gaps.
If you have already laid your floor, and not used air-entrained concrete mix, or a DPM, there is no point in pressing this advice on you. A damp-proof membrane (DPM) needs to form a seamless "bag" for the concrete slab, so must be laid on a sub-base of rubble or compacted crushed limestone ("MOT") whose surface must be blinded (made smooth) with sharp sand or hoggin. In a brick/block garage with a normal DPC (damp-proof course) the DPM can extend above the floor surface and then be covered with a brick or tiled skirting so that no damp brickwork is exposed. Your garage does not have a DPC, but the concrete from which the posts and panels were cast may be a strong enough mix to make it more or less waterproof.
As things stand with your job it sounds as if contributors who consider that the wetting of the surface of the painted finish is due to condensation are right. However, unless you used a mix for your concrete which should make it very resistant to capillary action I fear that the paint will sooner or later start lifting - unless you are on a very well-drained sandy soil, say.
AIR ENTRAINED CONCRETE
i owe what knowledge I have on this matter to the long-out-of print "Which? Book of Do It Yourself". This gives the "recipe" for paving mix concrete as an alternative, for a small job, to proper air-entrained concrete from a ready-mixed concrete supplier. You may still find copies of this DIY "bible" for sale on Ebay. My edition is from 1992. There was at least one further edition. After this, the Consumers' Organisation (Which?) unaccountably ceased publication of this, and all the rest of an equally invaluable range of DIY manuals.
