▁In ▁the ▁United ▁Kingdom , ▁it ▁is ▁common ▁for ▁composite ▁doors ▁to ▁be ▁cert ified ▁to ▁B S ▁P AS ▁ 2 3 / 2 4 ▁and ▁be ▁comp li ant ▁with ▁Sec ured ▁by ▁Design , ▁an ▁official ▁UK ▁police ▁initi ative . ▁ ▁A ▁steel ▁security ▁door ▁is ▁one ▁which ▁is ▁made ▁from ▁strong ▁steel , ▁often ▁for ▁use ▁on ▁v ault s ▁and ▁safe ▁rooms ▁to ▁with stand ▁attack . ▁These ▁may ▁also ▁be ▁fitted ▁with ▁wooden ▁outer ▁pan els ▁to ▁res emble ▁standard ▁internal ▁and ▁external ▁doors . ▁ ▁A ▁flush ▁door ▁is ▁a ▁completely ▁smooth ▁door , ▁having ▁p lywood ▁or ▁M DF ▁fixed ▁over ▁a ▁light ▁tim ber ▁frame , ▁the ▁h ollow ▁parts ▁of ▁which ▁are ▁often ▁filled ▁with ▁a ▁card board ▁core ▁material . ▁Sk ins ▁can ▁also ▁be ▁made ▁out ▁of ▁hard boards , ▁the ▁first ▁of ▁which ▁was ▁invent ed ▁by ▁William ▁H ▁Mason ▁in ▁ 1 9 2 4 . ▁Cal led ▁Mason ite , ▁its ▁construction ▁involved ▁pressing ▁and ▁ste aming ▁wood ▁ch ips ▁into ▁bo ards . ▁Fl ush ▁doors ▁are ▁most ▁commonly ▁employed ▁in ▁the ▁interior ▁of ▁a ▁dwell ing , ▁although ▁slightly ▁more ▁substantial ▁versions ▁are ▁occasionally ▁used ▁as ▁exterior ▁doors , ▁especially ▁within ▁hot els ▁and ▁other ▁buildings ▁containing ▁many ▁independent ▁dwell ings . ▁ ▁A ▁m ould ed ▁door ▁has ▁the ▁same ▁structure ▁as ▁that ▁of ▁flush ▁door . ▁The ▁only ▁difference ▁is ▁that ▁the ▁surface ▁material ▁is ▁a ▁m ould ed ▁skin ▁made ▁of ▁M