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A small, but important point that I'd suggest in addition to Alan's and Jimmy's suggestion for a good wash is a good rinse afterwards. I triple rinse mine afterwards, in warm and/or cool water, to be sure to remove any residual detergent. I find it crucial prior to undercoating, particularly as I use acrylic paint.
The old trick of mixing a bit of PVA glue with your paint may be a good idea in your case. Just a little bit applied as a wash before the undercoat or with the undercoat. You don't want it to become gluggy!