I suspect you may be brushing the tamiya fast set cement? The extra thin type cements are not to be brushed, instead, you hold the 2 pieces in position then dip the tip of the glue brush into the gap letting it flow via capillary action. It works by melting the plastic between the 2 pieces and join them together when set.
Having said that, the extra thin cements may not be ideal for 1/350 kits, esp the old ones due to age/mold deteriation there may not be a perfect mating surfaces/there are gaps. Any model cement will do in that case - these can be brushed on then press the parts in place (hold with tape if necessary) until dry.