Yeh, but it shouldn't ever really be a concern once the resin has cured. You should never mix two brands or different kinds of resin together while they are liquid or even partially cured, but once cross-linked they should be a hard surface just like any other thermoset polymer. Some may be harder to bond to than others, but I've never seen a properly cured thermoset resin react or cause a reaction with a similar or even completely different type of thermoset. 

one general rule with any RESINS and hardeners what you start with you carry on with !! never mix other brands with other brands !! not all manufactures use the same recipe's for there products that's why some are better for some things than others !!  

The temperature and humidity could well have caused your problem. 10C is too low for most epoxy systems.  20C is ideal and 15C is the absolute minimum in our experience. The lower the temperature, the longer the cure time.  The problem then as the cure time is longer, the resin is exposed to moisture for much much longer than normal during its cure. Before it is fully cured, Epoxy is very hydroscopic so at low temperatures, where the cure time is significantly extended, the resin then can absorb a lot more moisture than it normally would, to the extent that the resin can be damaged and may not fully cure properly.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thanks for your reply Warren. I mixed by weight using digital scales. The left over mix hardened fully in the mixing cup. I was working in my garage. Temperature was around 10 degC and humidity around 50 to 60%. I'll proceed with covering smaller areas and see if the same thing happens again. 

This is not a problem we have heard of in terms of incompatibility before.  As long as the west resin was fully hardened, then in terms of a reaction, the fresh epoxy does not react with the underlying material, it merely gets a mechanical grip on the material (hence the need to key with sandpaper).    The milkyness in the resin is moisture absorption into the resin in most cases. It could have happened and caused the issues or a secondary consequence due to the resin not fully hardening due to the other curing issues. What sort of temperature and humidity have you been working on?  Was the resin mixed by volume or by weight? 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I’ve been repairing a carbon-kevlar kayak. I had used alayer of West System 105 epoxy resin with 205 hardener but did not like all thebubbles that appear on adding the hardener and are v difficult to get rid of.After a cure period of more than a month I sanded to 80-grit and recoated witheasycomposites RapidRepair epoxy resin and fast hardener. When these are mixedthere are no bubbles – so preferable to the other brand!( I cleaned aftersanding with acetone but waited for that to dry before re-applying resin).Where I applied easycomposites resin on top of the West Systems resin, theeasycomposites resin did not harden fully and went slightly milky. I know I hadthe resin/hardener mix correct (100-33) because the mix hardened in the mixingcup and on parts of the kayak where there was no West System resin underneath.After waiting a couple of weeks for it to cure, I found it was still a bit stickyon the surface and although it had hardened it had the texture of polyethylenerather than a hard clear resin and could be peeled off with a paint scraperblade (luckily because it won’t sand off easily). So there was low adhesion tothe West Systems resin underlayer as well as the hardening issue. Has anyoneelse experienced this?